Take a look at this example in the online Vega/Vega-lite editor here. For local use, I have cloned and built both Vega and the editor according to the github instructions without problem. And there’s a lot of info at the main site. In your case, and for this area/density encoding, if your column is named value, I suspect the encoding for x will look something like this:
"x": {"field": "value", "title": "Value"}
which I believe is saying take the value of the datum in the Value column. This goes against what I’ve done in other plots, like a time series, where the x field is the name of the column. Guess it’s time to dig into the documentation on the main Vega site.