Dotted line on an X or Y-axis indicates that there are no cards on that axis (i.e. in that list or in that time frame).
Dotted border around a cell indicates that the card is due any time of the day (the time portion is “0:00”). Such cards are put first in the cell.
Gray line indicates a time frame in the past (e.g. yesterday). Pale blue line indicates a past Sunday.
Black line indicates a time frame which has not passed entirely yet (e.g. today or tomorrow). Blue line indicates a Sunday.