VOPlot – The
VOTable plotting utility (Version 1.2)

Creating data
subsets by applying filters
Statistical
Functions on Plotted Data
VOPlot (VOTable Plotting tool) is an applet for plotting different astronomical graphs using data stored in VOTable format. VOPlot is available in standalone version and a webbased version. The webbased version is integrated with the VizieR Catalogue Service and can be used to plot any catalogue by selecting output layout as "Plot (VOPlot)". The standalone version can be downloaded from http://vo.iucaa.ernet.in/~voi/voplot.htm.
Click here for Release Notes and disclaimer information.
About VOPlot
VOPlot has been developed as a part of the Virtual Observatory  India initiative by Persistent Systems and the InterUniversity Centre for Astronomy and Astrophysics (IUCAA), in collaboration with Centre de Données astronomiques de Strasbourg (CDS), with a support from the European AVO project. The collaboration between VOI and CDS extends to several related projects.
VOPlot uses Ptplot 5.2, a 2D data plotter and histogram tool implemented in Java. Ptplot has been developed at EECS department at the University of California, Berkeley.
The standalone version has same functionality as the webbased version except for a feature for saving the plot as an EPS file or as a VOTable file. In the webbased version, there is no menu to load a VOTable or to load multiple VOTables.
Standalone Version:
To use the standalone version of VOPlot, you will need to download the executable jar file named voplot.jar. The file can be executed by typing the following command at the command prompt:
java jar voplot.jar
This will open a java application window as shown in the Fig. 0. To use data from a VOTable you will need to load it first. You can either specify the URL of a VOTable or specify the path of a VOTable on your personal computer (PC) by clicking on the "open" submenu from the "file" menu. The message on the status bar will indicate whether the file was loaded successfully or not.
Note : To run VOPlot 1.2 you must have JRE 1.3.1 or higher installed. To see the version of JRE installed, type the following command at the command prompt:
java –version
Web Version:
1. Search for the required catalogue on VizieR, select a VOTable from the list and select the output layout as “Plot (VOPlot)”.
2. After clicking on "Submit Query" button the plotting applet loads. Note: Only numeric columns from the VOTables can be used for plotting.
3. Once the applet is loaded, you can see the numeric fields listed as columns in the dropdown list box.
To draw a plot of one columns against the other,
You can see the scatter plot. If required, you can plot data points on a log scale by setting the option appropriately using the "Log" checkbox.
Figure 2
To change the plot properties click on the " View > Plot properties" menu, which opens the “Set Plot format” dialog as shown in Figure 3.
For changing properties such as title, labels ranges etc., click on the "simple" tab.
Figure 3
For changing properties like color and marker style click on the "Advanced" tab.
Figure 4
To Load mutiple VOTables click on the "File > Load Multiple VOTables" menu. Select any other VOTable.
The two columns of the new VOTable which match the current plotted columns are overlayed and plotted on the current plot.
A new dropdown list appears on the right of the screen above the Yaxis dropdown list which allows you to switch between the VOTables.
A sample plot with two VOTables loaded is shown in figure 5 below.
Figure 5
To draw a plot with error bars, click on the “View > Plot with error bars” menu.
This will open the Error bars dialog. By default E_ABC(Upper case) or e_ABC(Lower case) are chosen as default columns for the error limits, where ABC is the YColumn.
If E_ABC contains the error then the error bar is drawn between ABC – E_ABC(The lower limit of the error bar) and ABC + E_ABC(The upper limit of the error bar).
You can choose any other columns as well as for upper and lower error limits.
The Error bars dialog is shown below in Figure 6.
Figure 6
A Sample plot with error bars is shown below in Figure 7.
Note : Error bars are only provided for the column on the Yaxis.
Figure 7
To draw the histogram of a column
A sample histogram is shown in Figure 8.
Figure 8
To plot the histogram bar heights on a logarithmic scale, check the "Log" checkbox corresponding to the Yaxis. If you want to plot the histogram of data points on a log scale, then check the "Log" checkbox corresponding to the Xaxis.
When drawing histograms on a logarithmic Yaxis, the bars with height 1 cannot be drawn. You can force the VOPlot to draw such bars by incrementing the height of all the bars by 1. This is done by checking the “Incremented Y” checkbox in the Histogram Properties dialog box
To change the histogram properties click on the “View > Plot properties” menu.
Click on the “Simple” tab to change properties like the bin width, X Label, Ylabel, Range, etc.
Figure 7
Click on the “Advanced” tab to change the color of the histogram.
Figure 8
For zooming into the plot you need to be in the Zoom mode. To go in zoom mode, click on the "Zoom" icon represented by or click on the “Zoom Mode” submenu from the “Mode” menu.
The "Zoom" icon represented by in depressed mode indicates that you are currently in Zoom mode.
To zoom in, drag the left mouse button down and to the right to draw a box around an area that you want to see in detail. To zoom out, drag the left mouse button up and to the left. To get the original graph click on the "Reset" icon represented by .
To take the mouse into select mode, click on the "Select" icon represented by or click on the “Mode > Select Points mode” menu.
The "Select" icon represented by in depressed mode indicates that the mouse is currently in Select mode.
To select points, drag the left mouse button down and to the right to draw a box, the data points that fall within the box area are selected. To select points all the data points on the plot, drag the left mouse button up and to the left. A square with green outline is drawn around the data point to indicate it is selected.
To take the mouse into unselect mode, click on the "Unselect" icon represented by or click on the “Mode > Unselect Points mode” menu.
The "Unselect" icon represented by in depressed mode indicates that the mouse is currently in Unselect mode.
To unselect points, drag the left mouse button down and to the right to draw a box, the data points that fall within the box area are not selected anymore. To unselect points all the data points on the plot, drag the left mouse button up and to the left.
To clear all the selection on the plot, click on the "Clear all" icon represented by or click on the “Mode > Clear all selections” menu.
To overlay plots (simultaneously viewing multiple plots with similar range on the same axes system)
You can see the second plot overlaid on the first one if the second one is in the same range as the previous one. Only that portion of the second plot is visible that is within the already plotted range.
An example of overlaid plots is shown in Figure 9.
Figure 9
To see the plots overlaid together (even if they don’t lie in the same range) click on the fill icon represented by . Figure 7 shows the complete datasets of all the overlaid plots in the graph above. A different marker will be used for each plot, to allow one to differentiate between the plots.
To overlay histograms (simultaneously viewing multiple histograms with similar range)
The second histogram is seen only if it lies within the same X and Y range as the first one. Click on the fill icon represented by to see both histograms irrespective of the range. A different color will be used for each histogram, to allow one to differentiate between the histograms.
Example of overlaid histogram is shown in Figure 10.
Figure 10
One can create new columns by defining transformations on them. You can use expressions with arithmetic operators, trigonometric functions, and miscellaneous functions shown below to create new columns. You can use transformed columns for plotting.
+ 
Addition 
 
Subtraction 
* 
Multiplication 
/ 
Division 
log(a) 
Log to the base 10. 
ln(a) 
Natural Log of “a” with base e, where e is Euler's number (i.e. 2.718...). 
pow(a,b) 
"a" raised to power of "b". 
sqrt(a) 
Square root of "a". 
exp(a) 
Returns the exponential number e (i.e. 2.718...) raised to the power of “a”. 
dexp(a) 
10 raised to the power of "a". 
cos(a) 
Trigonometric cosine of an angle. a = an angle in radians. 
acos(a) 
Computes the arc cosine of “a”. “a” must range from 1 to 1; the resulting angle is in radians and will range from 0 to pi. 
sin(a) 
Trigonometric sine of an angle. a = an angle in radians. 
asin(a) 
Computes the arc sine of “a”. “a” must range from 1 to 1; the resulting angle is in radians and will range from pi/2 to pi/2. 
tan(a) 
Trigonometric tangent of an angle. a = an angle in radians. 
atan(a) 
Computes the arc tangent of “a”. The resulting angle is in radians and will range from pi/2 to pi/2. 
toradians(a) 
Converts an angle measured in degrees to an equivalent angle measured in radians. 
todegrees(a) 
Converts an angle measured in radians to an equivalent angle measured in degrees. 
The dialog box for creating new columns is shown in Figure 9.
Figure 11
You can create new data subsets by defining filters on them. You can use a condition with relational operators and logical operators with operators and functions shown above to create new data subsets. Data subsets can be used for plotting.
< 
Less than 
<= 
Less than or equal to 
> 
Greater than 
>= 
Greater than or equal to 
== 
Equal to 
!= 
Not equal to 
&& 
And 
 
Or 
! 
Not 
The dialog box for creating data subsets is shown in Figure 10.
Figure 12
Once a data subset is created you can plot data from the subset. This can be done by choosing the data subset from the Filters combo box in the main applet window.
Note: "All" represents the complete data. Only data points satisfying the filter condition will be considered for plotting.
Statistical functions can be applied on the plotted data.
The dialog box is divided into two tabs – basic and advanced functions. The basic functions require only one data array, while the advanced functions require two or three data arrays as parameters.
The following statistical functions are currently supported.
Sr. No. 
Function name 
Input Columns 
1 
Number of observations 
X 
2 
Range 
X 
3 
Minimum 
X 
4 
Maximum 
X 
5 
Mean 
X 
6 
Variance 
X 
7 
Standard deviation 
X 
8 
Skew 
X 
9 
Kurtosis 
X 
10 
Linear correlation 
X, Y 
11 
Significance (t) for Linear correlation 
X, Y 
12 
Probability for Linear correlation 
X, Y 
13 
Rank correlation 
X, Y 
14 
Partial correlation 
X, Y, Z 
VOPlot, by default, takes only the only the plotted data range into consideration while evaluating the statistical functions. This will evaluate the statistical functions for the currently plotted columns, along with the applied filter, if any, in the current axes ranges. However you can force it to consider complete dataset by selecting the "Entire Data" checkbox. Similarly, you can force it to consider the selected data range by selecting the "Only Selected Point Data" checkbox
Sample dialog box with basic tab selected showing plot statistics is shown in Figure 13.
Figure 13
Sample "Advanced Functions" tab is shown in Figure 14.
Click on the “Box Plot” button on the “Plot statistics” dialog to display the Box Plot for the X column.
The box plot shows how the data is distributed relative to the four points, namely the maximum value, the minimum value, the 25% quantile and the 75% quantile.
Sample Box Plot is shown in figure 15 below.
Figure 15
Used to display Plot data in VOTable format. It also displays the filters and data sets that are userdefined.
To view the VOTable, click on “Data in VOTable format” submenu from the “View” menu. The "Display VOTable" dialog box will open.
The ‘Display VOTable’ dialog box by default displays the metadata and the data corresponding to the selected data points on the graph.
The entire VOTable data can be viewed by removing the selection from the checkbox “Display Data Of Selected Points”.
Note: The data displayed is truncated to the first hundred points.
Sample dialog box displaying VOTable containing data of selected points is shown in Figure 13 below.
Figure 13
The ‘Display VOTable’ dialog box by default displays the metadata and the data of the data points on the graph if points are not selected.
The selected points VOTable data can be viewed by selecting the checkbox “Display Data Of Selected Points”.
Note: The data displayed is truncated to the first hundred points.
This feature is available only in the standalone version of VOPlot. To save a VOTable, click on “Save As File” button on the dialog box. The save as dialog box will appear. Select a file and click on OK. The file is saved as an XML file, VOTable format. The various userdefined filters and data columns pertaining to all points (more than 100) are saved in the file.
Note: This feature is not
available in the webbased version of VOPlot. There is no “Save As File” button
on the dialog box as shown in Figure 14 below.
Figure 14
The VOTable data used to plot the graph can be displayed in tabular format. It also displays the filters and data sets that are userdefined.
To view the VOTable data, click on "Data in Table format" submenu from the “View” menu. The "Display Data" dialog box will open.
The data is displayed in two different tables. The first table displays the field metadata of the VOTable, while the second table displays the actual data.
If points are not selected on the plot, the entire plot data is displayed.
Sample dialog
box displaying the selected data and metadata of the VOTable is shown in Figure
15.
Figure 15
To view the data of the points selected on the plot. This dialog box shows data of selected points by default (if user has selected points); otherwise data of all points is displayed.
The selected points data can also be viewed by selecting the radio buttons “Show Selected”.
To view the data of all the points in VOPlot. If points are not selected on the plot, by default the entire data is displayed.
The entire data can also be viewed by selecting the radio buttons “Show all”.
In the standalone version of VOPlot, for saving the plot as an EPS, click on “Save As EPS” submenu from the “File” menu. The save as dialog box will appear. Select a file and click on OK to save the image as an EPS file onto your machine. Currently, the image can only be saved as an EPS file.
Note: This feature is not available in the webbased version of VOPlot.
For printing the plot click on “Print Graph” submenu from the “File” menu.
For feedback on VOPlot contact voindia@vo.iucaa.ernet.in.