/// <summary>
        /// This returns a string for specification of the image size in Gnuplot.
        /// </summary>
        /// <param name="arg"></param>expression (list), size in meters or if without unit, in pixels
        /// <param name="context"></param> 
        /// <param name="unit"></param> unit for gnuplot
        /// <returns></returns>
        public static string GetSizeString(Entry arg, Store context, SizeUnit resultunit)
        {
            TNumber Inch = Decision.NumericCalculation(Entry.Create(TermsConverter.ToTerms("'in")), context);
            TNumber size = Decision.NumericCalculation(arg, context);
            if (size.El(1).obj.Units.ContainsUnits() && size.El(2).obj.Units.ContainsUnits())
            {
                if (size.El(1).obj.Units.Text == "'m" && size.El(2).obj.Units.Text == "'m" )
                {
                    // compute the size string if given in m
                    if (resultunit == SizeUnit.Pixel) size = size /Inch * GlobalProfile.ScreenDpi;
                    else size = size / Inch;
                }
                else
                {
                    // complain about non-consistent units.
                }
            }
            else
            {
                // compute size given as pixels
                if (resultunit == SizeUnit.Inch) size = size /  GlobalProfile.ScreenDpi;
            }
            // Extract list entries to string
            return size.El(1).obj.ToString(2, 10, FractionsType.Decimal, false) + "," 
                +  size.El(2).obj.ToString(2, 10, FractionsType.Decimal, false);
        }