As with any screen of data, room on the screen may be precious. When you are plotting multiple sets of data, the legend of the plot chart is important to show what metrics are being plotted. However, the legend may take up quite a bit of room on the screen. How can you condense this? One way is to use a collapsible legend to show and then hide the chart legend.
Here's an example (this example is taken from Ernie Gilman's excellent Networks_Extended workspaces). Here we see a plot chart for the network byte rate for multiple TCPIP stacks on z/OS.
If you look at the bottom left corner of the chart you see the legend which shows which plot line is for which TCPIP stack. To make the legend go away, click on the triangle pointed at by the arrow.
How do you enable the option? If you look at the properties for the chart and click the style tab, you see an option "Place legend in collapsible panel". Check this option to enable the collapsible legend.
It's an interesting technique to save a few pixels of space on the screen.