You can change the default decimal separator, set by the default CultureInfo settings of the computer, to anything else that you want, using code similar to below:
System.Globalization.CultureInfo cultureinfo =
new System.Globalization.CultureInfo (System.Threading.Thread.CurrentThread.CurrentCulture.Name);
cultureinfo.NumberFormat.NumberDecimalSeparator = ".";
System.Threading.Thread.CurrentThread.CurrentCulture = cultureinfo;
Place this in the constructor of your main form so that it is called before any plotting routines.
There are many variants of this that you can use (just search on the web), but we had problems with ReadOnly access of the CurrentCultureInfo settings. The code we show above seems to work.