Of the metrics the program can plot, only some show up as tabs on the panel out of the box. You choose which appear from the metric tab strip below the chart.
Right-click the metric tab strip (the row of metric names below the chart) on the tab whose metrics you want to edit (Servers, User sessions, or Processes). The menu lists every metric the program collects for that entity type, each with a checkbox - tick or untick a metric to show or hide its tab directly.
The list differs by tab - Servers, User sessions, and Processes each expose their own metrics:



For the same choices in a dialog (with Select all, Clear, and Defaults buttons), choose Graph settings... from that menu, or Servers / Users / Processes > Graphs > Graph settings... on the main menu while the matching tab is active.

A ticked metric becomes a tab in the panel for that data tab. An unticked metric is hidden; ticking it later restores the tab. The metrics themselves are always collected; the dialog only controls visibility.
The defaults are tuned for "the numbers most people watch first". Add the niche ones (frames skipped, FEC rate, encoding time) when you need to dig into RDP quality. To change the defaults that every newly opened chart starts with, see graph options in Preferences.