`gwl_area_plot()`: Change scale breaks for wells with short records

For gwl_area_plot(), perhaps consider changing the x axis to yearly breaks when there are fewer than 10 years (or fewer than 6 years) of well data? This isn't a very common occurrence, but would improve the legibility of those figures.

I'm more than happy to create a pull request if you think this would be a good thing to include.

Start by locating the gwl_area_plot() entry point and inspect how x-axis breaks are selected for well records. Compare the current plot behavior for records shorter than the proposed six- or ten-year thresholds. Done means short records use yearly breaks and the resulting figures are more legible; no file or test is named in the issue.

Written by the indexing model from the issue text.