Chicago county cases report zero until they have data

[sglyon]

From CAN

For 2020-03-17 to 2020-05-03 valorum has total tests 0, then it jumps to 79,625 on 2020-05-04. Cases are > 0 during this time so using the simple formula of negative tests = total tests - cases we end up with negative cases

Explanation

We are getting our chicago data from https://www.dph.illinois.gov/sitefiles/COVIDHistoricalTestResults.json?nocache=1

They are actually reporting identically 0 instead of missing for the dates until May 04.

Proposal

I propose we check each date for a positive state counts and 0 for all counties set the county values to missing for those dates