Line Chart drops to 0 on missing dates (Sparse Data) - How to interpolate?

My suggestion is to use a bar chart format. 

Note that with Many Chart Types like line and bar charts, there will be a legend at the bottom for the series values, for example, your risk types. Hovering over one of those will highlight that particular risk type and kind of fade out the other risk types so you can both see all the Risk together, which might be a little jumbled, but you can also easily do a hover over Risk A and see the bars just for Risk A.