Bug where the graphing function clears the stack

[c3d]

trafficstars

While testing tvars commands in #924, I ran across a case where the stack rendering code was crashing because reading the stack would read zero.

Investigating showed that the stack had been cut by the graph rendering for an equation, presumably because we are consuming more from the stack than is available. We need to have protections against this in the grapher.

[c3d]

The bug occurs because the test was using bar as a variable name, which happens to be a shortcut for barplot, which is a command with a non-zero arity.