fix: Performance improvement for scattergl with many points. Issue #7065

[giuseppe-straziota]

The performance issues are always intriguing, and I tried to determine why the browser freezes when handling a large number of points. I believe the problem stems from two main factors.

The first factor is the hover effect, which triggers a function that calculates the distance between the mouse pointer and each point in the set. This process can certainly contribute to the performance problem, I believe the best improvement would be using a filter calculated by a window determined by the pointer coordinates with a delta, in this way, we can calculate the distance only for a subset. One possible improvement could be using squared distances for comparison, thereby avoiding the computation of square roots, similar to the method utilized by the K-d tree algorithm, if the plot allows it.

The second factor involves a loop in the part of the code I modified, where "newDistance" is created for every point. This results in excessive overhead for the garbage collector, causing the browser to freeze.

for(var i = 0; i < cd.length; i++) {
            var newDistance = distfn(cd[i]);
            if(newDistance <= pointData.distance) {
                pointData.index = i;
                pointData.distance = newDistance;
            }
        }

I moved the variable outside the cycle and used a precalculated value for the array length. I conducted tests in the development environments provided by Plotly using the test_dashboard tool.
The values represent average times measured in milliseconds..

	6000	10000	100000	1Milion	10Milion	20Milion
inside for	0,841015625	1,892871094	17,34858398	154,4843262	1412,1573	2733,622534
outside for	0,7019287109	1,51418457	13,50952148	113,6539307	1057,211401	1078,00

The red line shows the performance for the variable inside, the blue line outside. The vertical axis (Y-axis) represents the time measured in milliseconds.

Of course, the performance depends on the engine and the environment in which it runs, but I think this conveys the idea of performance.

Please let me know if this can be the first step to improving performance.

[gvwilson]

@archmoj please have a look after the geojson stuff is sorted out - thank you

[giuseppe-straziota]

I apologize, everyone; I mistakenly referenced the wrong issue. The correct issue number is #7065 sorry.

[archmoj]

Excellent PR. 🏆
Many many dancers:
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[giuseppe-straziota]

I'm glad to have been helpful :)