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Chapter 8 Plots

2. Cartesian Plots

Cartesian plots graph values in a 'cartesian format.' Cartesian plots are almost always used to plot the values of array channels such as the VAL fields of waveform and compression channels. Trying to plot other types of channels is possible but difficult. They can plot up to eight different y channels against one x channel; however, you can omit the x channel, whereupon x will be either the 'bin number,' the number of waveform elements, if the y channel or channels are fields within a waveform channel, or a 'sample number' if the y channel or channels are connected to a non-waveform channel.

The cartesian plot's property sheet has two unique properties: the 'plot style' property, the 'sample save count' property and the 'erase oldest sample' property. The 'erase oldest sample' property performs no real function, so you can ignore it.

The 'plot style' property has three options: (1) point plot, (2) line plot, and (3) fill under; however, the fill under option has not yet been implemented. A cartesian plot is determined by data points, a data point being the point in time in which the x and y values are sampled and in which a coordinate in the graph is plotted at the intersection of the x and y values. The point plot option just leaves the data points to be displayed as they are, while the line plot option draws lines between them.

The 'sample save count' property performs two functions. When set to zero, DM will update the plot only with the most current sample, erasing all other samples. Before each new sample is plotted, DM erases all other samples, thus clearing the plot. The second function occurs when set to any positive number greater than zero. When the property is set to any positive integer greater than zero, DM will not clear the plot before updating the plot with the current sample, so DM keeps updating the plot sample after sample with the old samples visible as well as the most current one. In addition, when an X Window operation is being performed on the screen or when the display with the plot is hidden or iconized and DM does not update the plot, DM will store samples in the circular buffer. DM will update the plot with these values when the display window is re-exposed. The number of samples it stores equals the number specified in this property.

Figure 8-2 shows an example of a cartesian plot that plots three waveform channels (the y axis) against a 'bin number.'

EDD/DM User's Manual, 2.4 - 27 MARCH 1997
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