I claim the ornamental design for a towel bar, as shown and described.
FIG. 1 is a perspective view showing my new design for a towel bar;
FIG. 2 is a front view of FIG. 1;
FIG. 3 is a rear view of FIG. 1;
FIG. 4 is a side view of FIG. 1;
FIG. 5 is another side view of FIG. 1;
FIG. 6 is a top view of FIG. 1; and,
FIG. 7 is a bottom view of FIG. 1.
The broken lines in the drawings illustrate portions of the towel bar which form no part of the claimed design. The towel bar is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.