Alternative histogram visualisation

[gomoripeti]

Add option to show histogram (buckets) instead of percentiles on the graphs to better understand distribution.
This is not only useful for latency but even more for distribution of other values (eg argument).

The ideal visualisation would be something like a heatmap or spectogram of sysdig.

• http://www.brendangregg.com/heatmaps.html
• https://sysdig.com/blog/aws-storage-latency-sysdig-spectrogram/.
• https://docs.datadoghq.com/graphing/dashboards/widgets/
• https://www.circonus.com/2012/09/understanding-data-with-histograms/

Time should be on the x axis for consistency with the percentile graph.
Experiment whether it still makes sense to overlay total call count on left-y axis.
Unfortunately c3-js does not support heatmaps out of the box so this needs to be implemented manually based on d3. (or find another lib)

This is only the frontend story assuming that the backend sends data in appropriate json format.