Auto-captions for gramexcharts

[jaidevd]

Use the text generation framework:

chart data structure -> analysis metric -> calibration -> entities -> template

to generate captions for the following types of charts:

• [ ] bar charts (compare a variable to a fixed value - min, max, mean, median, percentile, etc.)
• [ ] scatter plots (infer a relationship between two variables)
• [ ] donut / pie charts (part of whole comparisons)