For the bar mark, the domain is ordinal, which implies that each date gets a tick, and a missing date is skipped.
Instead of barY one can use an areaY mark, with curve: “step” to give it the same shape.
Alternatively, if you still want individual bars, “bin” the dates by day and use rectY marks. But in that case the bars might not be wide enough to be well displayed, since there are many days.