The ornamental design for a protractor for a chalkline, as shown and described.
FIG. 1 is a perspective view of a protractor for a chalk line, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is an another perspective view thereof; and,
FIG. 9 is an operational perspective view thereof, wherein the broken lines are shown for illustrative purposes only and form no part of the claimed design.