ext/tk/lib/tkextlib/tcllib/plotchart.rb: pixel_to_coords should invoke pixelToCoords, not coordsToPixel.

This pull request changes 'Tk::Tcllib::Plotchart.pixel_to_coords' and 'Tk::Tcllib::Plotchart::ChartMethod#pixel_to_coords' to invoke the appropriate function of Tcl.