PR: Improve xticks and xtick labels plotting in hydrographs

[jnsebgosselin]

In my opinion, the way the xtick labels are currently aligned with the xticks is confusing, i.e. the top left corner of the labels is aligned with the middle position between the major tick marks.

Also, I think that using major xticks and enabling vertical grid for xticks that do not have a label make the graph too dense.

So this PR introduce the following changes to address the aforementioned issues:

• Aligned xtick label with the major xticks.
• Use minor ticks when there is no label.
• Don't use vertical grid for minor xticks.

For comparison, this is what the xticks and xtick labels look like before this PR.