add feature_histogram() function

[3r3n-n]

Hi!

I would like to propose this implementation for the chart.feature_histogram() function (addressing issue #339).

This implementation was manually and visually tested against the GEE JavaScript API example (see images below), Tooltips also work.

It was necessary to import the math library in order to implement the nextPowerOf2() function that is required for adjusting the value of maxBuckets and minBucketWidth in a similar way to what the GEE documentation says.

The start_bins and end_bins variables were added to make nicer midpoint offsets and make them as similar as possible to the Google Charts histogram output.

This function takes the same input arguments as the GEE JavaScript API equivalent... except for maxRaw. I have not found a way to incorporate an equivalent parameter with bqplot. So far, I have not found a case using maxRaw in the JS API function where the same histogram cannot be reproduced with this function in jupyter (without any maxRaw-like input). From the information available in the GEE documentation, I understand that maxRaw only affects the output when calculating a reducer value (but I might be wrong). So the current version of this proposal considers that maxRaw is not necessary for displaying the histogram in Jupyter.

I would like to know if this approach is adequate or if there are some other things that I should take care of, in order to be considered for merge.

These changes passed tox tests for python 3.5, 3.6, 3.7, 3.8.

micro-demo

maxBuckets = None, minBucketWidth = None

JavaScript

Python (this function)

maxBuckets = None, minBucketWidth = 0.5

JavaScript

Python (this function)

maxBuckets = 30, minBucketWidth = None

JavaScript

Python (this function)

maxBuckets = 30, minBucketWidth = 3

JavaScript

Python (this function)

[giswqs]

Excellent work! Could you also add a notebook example (e.g., 79_chart_histogram.ipynb) and put it under https://github.com/giswqs/geemap/tree/master/examples/notebooks. That would be very helpful for geemap users.

[3r3n-n]

Done :) Just added the notebook example.

[giswqs]

@3r3n-n Looks great! Thank you very much for your time and efforts. Much appreciated.

[giswqs]

The notebook example has been added to the website https://geemap.org/notebooks/79_chart_histogram