Map improvement: lat/lon grid line density and labels

[huiwanpnnl]

I am guessing that currently, our code is hardwired to show lat/lon grid lines at 30 degree intervals. If I crop the global map to a small region, I may end up having no grid line (or only the boundary) — and no coastline — in the viewport, making it hard to tell which part of the global map I'm looking at right now. The screenshot below is an example. Here, the lat/lon cropping control panel and the variable names are providing some information about the geographical region, but if we could

• always show at least 3-5 longitude lines and 3-5 latitude lines (boundaries included) inside the cropped region, and
• show some ticks and labels (for latitude and longitude) along the perimeter of the cropped region,
  That would be very helpful. Thank you!