Axis origin jumps around once nsPerPx approaches 1

We represent timestamps as 64 bit integer nanoseconds. Axis tick marks and the origin can only fall on timestamps. As we draw the axis starting from x=0, once we have fewer nanoseconds to display than we have pixels it is generally not possible to display the origin at exactly the pixel we wish to, as it won't correspond to a timestamp.

We should probably draw the axis around the origin instead.