You know which days had valid measurements. Pass the time coordinate corresponding to those days as the "x" coordinate to plot, if you want a line plot that draws between the missing data.
stem() plot might make sense in some cases.
It is possible to draw in segments. For that purpose, pass in nan as the y coordinates for the missing measurements, and be sure to include a marker type. You want the marker because you might have patterns such as NaN Point NaN and plot() will only draw a line when there are at least two finite points in a row.