ts-plot-sar function plot_blockdev() silently fails on some versions of sysstat

Some versions of sysstat report block device data with this header:

# hostname;interval;timestamp;DEV;tps;rd_sec/s;wr_sec/s;avgrq-sz;avgqu-sz;await;svctm;%util

while the function plot_blockdev() (lines 160 and 162) expects the throughput columns to be "rkB/s" and "wkB/s" respectively, and gnuplot fails to find these columns. plot_blockdev() then exits immediately and no blockdev plots are created.