Graphs update every day at 9, 12, 3, 5:30, 8 and 11:30 pm (updates are usually done by 5-5:30 pm, and often earlier, but there have been occasional updates to the county page later than that). Occasionally the county updates information more than once in a day, especially on Mondays, when they often update in the morning with weekend data, and again later with new Monday data. As of 11/27, the county website will not be updated on Saturday and Sunday. More information at the WW county website.
The graph below shows the number of cases as a percentage of all tests; each data point is this percentage over the 7 day period ending on the corresponding date, that is, new cases divided by new tests for the previous week. For the period 3/21 to 5/28, positive cases were 4% of all tests.
In early August, the the Washington State Department of Health stopped reporting negative tests so they could change the way they count. Until then, they were not actually reporting negative test results (though that is what they called them)--they were reporting the number of people who tested negative, so a person who tested negative twice, even months apart, was reported as one negative test. Negative tests are now being reported again, so I've restarted the graph.
The spike around 11/20 seems to have been due to a delay in updating negative tests for a few days.
Again, as of 11/30, the negative tests number has not been increased since 11/23, so the positive rate has shot up to 100%. When they start reporting again, I'll fix the graph.