gp­data – Cause Meta­Post graph pack­age to ig­nore '#'-com­ments

This file mod­i­fies the Grdln_ rou­tine of the Meta­Post graph pack­age, so that it can parse and ig­nore com­ment lines in data files start­ing with a ‘#’ sign, sim­i­lar to the con­ven­tions in Gnu­plot.
Note that the graph pack­age will still in­ter­pret an empty line as “end of data” (even if it pre­cedes any data).