First, you can set the size of a graph exactly (as well as most of the offsets) in actual units of measure (in, cm, mm). See How to Specify the Size of a Graph for details.
In places where you can input a pixel offset, you can also use units of measure. See this example where the size of the graph, the margins, and the offset for this Text command are all entered in inches.
For an actual conversion, the pixels are 1/72 of an inch. This is based on the standard established by apple. Now, we really should probably refer to these as points, as the screen pixel no longer matches the standard.
For more information:
https://en.wikipedia.org/wiki/Dots_per_inch#Computer_monitor_DPI_standards
So in DataGraph, if you enter ’72’ for pixels, or ‘1in’, you get the same size.
Hope this helps! Sorry for the confusion. We will work on improving how this is documented.