Hover select year and tooltips

[anqi-lu]

• [x] In the multi-line chart, hover will show a vertical line representing the year being hovered over.
• [ ] Make the hover line only hover on discrete years instead of continuous
• [x] It will also trigger a tooltip showing the word count for selected keyword(s) for that given year.
• [ ] Make a box to display the keyword and count

Like in Curran's StreamGraph: https://unhcr.github.io/dataviz-streamgraph-explorer/#types=1-2-3-4-5-6-7

[bpartopour]

This is so cool, I can use a similar approach for my project, year selection!