Cannot plot a singleton point with plot in Tcl

[rwoldford]

The TCL plot function fails when given a single point to plot.

For example in R,
l_plot(x = 1, y = 1)

Produces
Error in structure(.External(.C_dotTclObjv, objv), class = "tclObj") :
[tcl] key "" not known in dictionary.

(the above happens in Python call too), plus R adds
Error: $ operator is invalid for atomic vectors

This appears to be a bug in the loon tcl code.

[rwoldford]

Fixed by Zehao