So the first suggestion is to add a mean of 50/100 line to the graph.
After 6 months solving about 100 cubes a day, the graph looks like this (times are made up):
Adding a mean of 100 red line, the graph looks like this:
See? The black dots are enough to show improvement.
The next suggestion is to distribute the times evenly along the x axis.
I deleted the third week's times to show why it's not a good idea.
In the above graph, you can clearly see that some data is missing.
In the above graph, you can't see that data is missing and there's an "unnatural" sudden drop in times (red arrow).
I'm not sure I understand your suggestion about time frame adjustment. If you want to see statistics about the whole history, select all times. If you want to see statistics about the last month, select only the last month's times.