When the plotting method (e.g. plot2d) is called, Chart::Gnuplot would generate a Gnuplot script based on the information in the chart object and dataset object. Then it would call the Gnuplot program. Unless specified explicitly in terminal of the Chart object, Chart::Gnuplot would by default generate the image in PS format first and then convert the image (by ImageMagick) based on the extension of the filename. The rationale of this approach is that the postscript terminal is so far the best developed teriminal and so this would let users to enjoy the power of Gnuplot as much as possible.
 
Because the default terminal is postscript, if ImageMagick is not installed, you would always need to specify the terminal if the output format is not PS (or EPS).