Redefine barchart tooltip: more understandable tooltip

[noelchen90]

Solves #426 bullet point 14.

14. They (test users) were not familiar with some of the metrics. They would love to have more explanations (e.g. what is lexical diversity). (proposed by Ying)

• adds y-axis label name
• modify tooltip format

The look Before

The look After

[pfliu-nlp]

Thanks!