Improve line graph tooltip

[yubiuser]

What does this PR aim to accomplish?:
It does 3 things:

1. While thinking about Total Queries Graph - Summary hover screen #3600 I noticed that the tooltip in the total query graph was floating above the cursor position and mostly hiding the bars. In the client activity this was a bit better, because the tooltip was below the cursor position but it would still follow the cursor in y direction which could also hiding bars. This PR fixes the tooltip to the x-axis independently of the y-postion of the cursor.

2. The client graph tooltip did hide all elements which had no queries in the selected time slot, the query graph did not. This is fixed now

3. Fixed the calculation of the tooltip percentage (this was most obvious in low-query time slots, see the screenshot above). The issue was that the total sum of the time slot was calculated by the values of tooltipLabel.parsed._stacks.y, but it did not only contained key/values for the actual data but some meta-data. This is fixed by only sum the values for keys not starting with _

Fixes #3600

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