@pierjsap He's talking about the old /watch_list.shtm page. But, it's good advice when making your own charts from watchlists in the new UI!
I would expect that's caused by an NaN or Infinite value in one of the point's series. Do most of them chart individually if you uncheck the other boxes adding those points to the chart? Or, perhaps easier, if you download the data using the green download arrow after the chart has not rendered, do you find NaN and Infinity in the data set?
Otherwise I would wonder if there was a message in your logs about what could have failed. But, it looks like the chart generated without showing any values, as in these images where I have a script doing,
I produced these charts: