[Feature Request]: Adding ability to plot lines in TensorBoard

[rsandler00]

Hey,

Huge fan of TensorBoard! However, a huge gap of TensorBoard is its inability to plot lines. Current summary objects are scalars and 2D images. What is clearly missing is an ability to plot 1D vectors, aka lines.

This is very useful when it comes to plotting PR curves and ROC plots, and probably a ton of other case-specific needs.

This is different than using an image summary to plot a 1D vector, since it would show both x-axis and y-axis of the values, be hoverable, and be in 2D rather than a line (try plotting a PR curve as a single row image).

The only way I know to save PR curves currently is to first plot the PR curve as a Matplotlib image and then save the image, but this is clearly very inelegant and wastes a bunch of space. Furthermore, the TensorBoard PR-Curve plugin is specific only to PR Curves, and has not been updated to TF 2.0.