Changed graph/Plot utc_offset from short to int so it doesn't overflow for GMT>9

Since the range of short is -32768 to 32767, time zones greater than about GMT+9 or GMT-9 cause this value to overflow. 

The result is that the GNUPlot graph generated has the wrong timestamps.

For example, testing from Sydney, Australia (GMT+10), this offset is 36000 (10 * 60 * 60) seconds. Casting to short overflows this value to -29536 (which looked like the GNUPlot graph was drawing timestamps for some US timezone, approx GMT-8)