Anybody have a good solution for an indicator card that shows which days are missing data (sales)?

In some cases like this you have to join your sales data with a calendar dataset that has all days in it. That way you'll have records of days with no sales data, but at least something is there when you have no sales.

Luckily the horizontal line and bar charts are pretty good at representing the same thing without that extra work.

Take, for example, a chart I have attached with employee punch data.  Nobody worked Sunday but the chart still shows the day.  This would be a pretty simple example of what you could do.