SMath Studio version: 0.99.7808 and previous stable release.
OS: Windows XP, Windows 7.

The plug-ins X-Y Plot and ZedGraph do not plot constant values.

Pop-up error message:
"The specified argument is outside the range of valid values.
Parameter Name: arrayIndex"

Constant value do not plotted in both cases:
as a number or as a variable name.

The only way to plot it is to define constant function y(x):=const
and then plot the y(x).

In the earlier releases of SMath Studio both plug-ins worked similar to the standard 2D plot.

Your version has to be doctored or use as previous versions.

It is not about a version, but about the settings.
Issue is reproduced and will be fixed asap.

Best regards.

Fixed. Fix will be a part of the next release.