Hi and thanks – and sorry for not getting to this sooner 
We don't currently have this on the immediate roadmap and this is actually the first time this request came up. I guess so far we've mostly been focusing on more classic machine learning / "extracting structured information from unstructured data" workflows, so time series annotation is a bit different.
In theory, you could probably implement something like this yourself, but I'm not 100% sure how involved this will be. For instance, here's an example of embedding a Vega chart using custom HTML and CSS: Embedding vega charts - #2 by ines This definitely lets you render the visualization – if it also lets you draw on it or select points, you could use Prodigy's update function in JavaScript to update the JSON with whatever you selected in the UI. It's also possible that a different charting library like chart.js would be a better fit, so if there's a library you know of that works well and looks good, I'd definitely be interested 