Which pchar numbers generate which plotting characters? In general, there are two ways to get an answer to a question like this: issue the command help, or help graph if you want help on graph (also try help introduction!); to open a hardcopy manual (several copies should be available from your instructor); or to use the on-line help facility, the electronic version of the hardcopy manuals.
The only ``exceptions'' to this rule are the five things covered by the display command: fonts, special characters, hatch patterns, line types, and, yes, the plotting characters. For example, to get a table of possible pchar symbols and their codes on your screen you would use display pchar rather than help pchar; the latter simply gives you a brief description of the syntax of the pchar command but not the table of codes.
There are two manuals for plotdata: one is the User's Guide which takes the same tutorial approach that we have been using here, in much more detail, of course; it guides you by showing many examples. Once you have read the introductory chapters, the best way to use it is to flip through its pages until you find an example that looks like something that you have in mind, and use the commands of that example as an inspiration for generating your own graphs. The other is the Reference Manual which is the detailed description of all commands and their options.
Returning back to our original question: as an alternative to display pchar you can look up the table of pchar symbols in the Reference Manual on page 140.