#51 Lower sample rate displays wrong values

[Richard Farthing]

Custom module - if the sample rate is lower than 1 min (to reduce CPU load on e.g Raspberry pi), then the 1hr graph wobbles up and down, it appears like there's a rounding error. It does the same on an x64 server on older version of webminstats also, and with other modules.

Example shown attached when I change the sample rate to 2 mins at 17.45 (same effect if it's 5 mins - my usual default). The range shows as +/- 0.06 when sampling 1 minute (correct), but +/- 0.25 or more when sampling at lower rates, which is wrong (I checked the 1 min samples in the rrd dump).