Get Nearest Touched Spot Slow for Large Datasets

The `getNearestTouchedSpot` function is really slow for large datasets (think 70,000 data points).

When snooping through the code, I noticed that when searching for the nearest spot, the LineChartPainter iteratively builds a list of all the spots in every trace. Expanding a list can be quite expensive, especially if you are only trying to track a single value.

Since this is a relatively easy fix, I will file a MR shortly

Start by locating getNearestTouchedSpot and the LineChartPainter implementation, then trace how spots are collected for each trace. Benchmark or profile the nearest-spot search with a large dataset and verify that it avoids building unnecessary lists while returning the same result.

Written by the indexing model from the issue text.