Fix/just workload in workload graph

[L-M-Sherlock]

source: open-spaced-repetition/fsrs4anki#686 (comment)

[Expertium]

You can use this for smoothing:

from scipy.signal import savgol_filter
cost_per_memorization = savgol_filter(cost_per_memorization_raw, 10, 2)

10 is the number of datapoints in a rolling window used for smoothing, 2 is the degree of a polynomial that is used to fit the data. But this requires a lot of fine-tuning, and doesn't make the graph perfectly smooth.

[L-M-Sherlock]

What about changing the thresholds of different color areas?

[Expertium]

    target2 = 2 * min_w
    target3 = 3 * min_w

Change these to these:

target2 = 1.5 * min_w
target3 = 2 * min_w

EDIT: ok, there's more. I'll submit a PR later. Finish this PR first.