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I used SigmaPlot in the past and liked it a lot.  

If you are happy with the look of Excel charts, click on the graph to have 
it selected, then go to "File-Page Setup". In the "Chart" tab, select 
either "Fit page" or "Custom", not the "Use Full Page" option, if you want 
to preserve the aspect ratio. 

Then print the chart to a PDF file or a postscript file. If you have Adobe 
Acrobat installed, "Adobe PDF" will show up as a printer name. When 
printing, look at the printer properties and set the "Adobe PDF conversion 
settings" to "Press quality". This will give you 2400 dpi resolution,  

If you do not have Adobe Acrobat, you can install a printer driver for one 
of the postscript color high-resolution printers (e.g., QMS Magicolor), or 
the "generic Postscript printer". Then when you print, choose "print to 
file" option. This will generate a postscript file (usually at 1200 dpi) 
that you can open in Photoshop and save as a TIFF. (or use some the ps2pdf 
or ps2tiff conversion utilities if you have a Linux system around).

Stan