A.3.6 Plot Window

Plot Window

Plot Window

The plot window allows you to plot the values in two table columns against each other. You can display it using the Plot () button when the chosen table is selected in the Control Window's Table List.

On the plotting surface a marker is plotted for each row in the selected Row Subsets at a position determined by the values in the table columns selected to provide the X and Y values. A marker will only be plotted if both the X and Y values are not blank. If more than one subset is being plotted, they will be drawn using different markers. A key on the right hand side indicates the marker being used for each subset. The marker types can be changed using the Marker Types menu.

You can zoom in and out of the plot by dragging with the left mouse button down and right (zoom in) or up and left (zoom out) - this takes a little practice but is easy to use after a couple of goes. If you get lost you can push the Rescale button () to return the scaling to normal.

Below the plot there are two sets of controls for selecting the table column which will provide the X and Y axis values. Each one consists of two parts:

Column selector
A selection box from which column names in the main table can be selected. Only columns which can be plotted from (i.e. scalar numeric ones) will be displayed in this selector.
Log checkbox
This checkbox can be clicked to toggle whether the axis in question is to be plotted logarithmically or not. If it is logarithmic, any negative values are simply ignored (not plotted).
Flip checkbox
This checkbox can be clicked to toggle whether the axis in question is to be plotted reversed. Normally (unticked) X axis increases left to right and Y axis increases bottom to top. When ticked, X axis increases right to left, and Y axis increases top to bottom.

To the right is a set of checkboxes headed Row Subsets. Click on these to choose which of the table's defined Row Subsets should be plotted on this graph. Different subsets are plotted using different markers, so you can see where different groups of results lie in relation to each other. You can alternatively use the Subsets To Plot item on the Subsets menu. The subsets are plotted in order of which was most recently selected. This makes a difference on a crowded plot or where some points are members of multiple subsets, since the most recently plotted symbol will appear on top. If points from one subset are being hidden behind those from another, you can deselect and reselect that subset and they'll be shown on top.

The following extra buttons are available on the toolbar:

Export as EPS
Pops up a dialogue which will print the current plot as an EPS file. In general this is a faithful and high quality rendering of what is displayed in the plot window. However, if plotting is being done using the transparent markers, it won't come out right since transparency cannot be represented in PostScript; the markers will be rendered as if they were opaque. Currently, if there are many points being plotted, this can result in a rather large output file.
Export as GIF
Pops up a dialogue which will output the current plot to a GIF file. The output file is just the same as the plotted image that you see. Resize the plotting window before the export to control the size of the output GIF.
Rescale
Rescales the axes of the current plot so that it contains all the data points in the currently selected subsets. By default the plot will be scaled like this, but it it may have changed because of changes in the subset selection or from zooming in or out.
Replot
Redraws the current plot. It is usually not necessary to use this button, since if you change any of the plot characteristics with the controls in this window the plot will be redrawn automatically. However if you have changed the data, e.g. by editing cells in the Data Window, the plot is not automatically redrawn (since this is potentially an expensive operation and you may not require it). Clicking this button redraws the plot taking account of any changes to the table data.
Grid
Toggles whether a grid is drawn over the plotting surface or not.
Draw Subset Region
Allows you to draw a region on the screen defining a new Row Subset. When you have finished drawing it, click this button again to indicate you're done. See Appendix A.3.6.1 for more details.
Subset From Visible
Defines a new Row Subset consisting of only the points which are currently visible on the plotting surface. See Appendix A.3.6.1 for more details.

The Marker Types menu allows you to select a set of markers which will be used for plotting. Some of these sets are marked "Transparent" - for these, instead of pixels on the plot blocking out ones already plotted, the more markers that are plotted at a given screen position, the darker in colour it will appear. This can be useful if you have very many points to plot, since you can see by the colour of pixels on the plot how many points are there in crowded regions. Unfortunately transparent points are not rendered properly when exported to PostScript files (they come out opaque), but they still work when exported to GIF format. The marker type set used initially depends on how many rows there are in the table (large dots for few rows, small ones for many).

The Regression menu provides facilities for calculating and plotting linear regression lines for some or all of the subsets on display. The following options appear on the menu:

Plot Regression For Subsets...
Presents you with a checkbox menu which allows you to select which subsets regression lines will be displayed for. Each subset whose box you select will have a line plotted on the graph representing its least-squares fit regression line in the same colour as the points for that subset.
Display Regression Coefficients
Displays a window giving the gradient, intercept and product moment correlation coefficient for each regression line which has been plotted.
Note the regression lines plotted are those calculated from all the visible points in the subset in question - any points off the edge of the graph are disregarded. Thus, zooming in and out will change the correlation coefficients and line geometry. The linear correlation functionality is experimental in this version of the program, and will be improved in future.