Y-axis tick input format

[dan-knight]

These changes replace the yaxis1.interval and yaxis2.interval parameters with a single yat input. This is meant to follow the standards set by BPG. If yat is not provided, the existing default will be used to set up evenly spaced ticks. yat should be a list of vectors corresponding to each axis. (yat[1] will be used for the first Y axis, and yat[2] for the second.)

For example:

yat <- list(
    seq(0, 100, 10)
    seq(0, 6000, 1000)
    )