The ornamental design for a diningarmchair, as shown and described.
FIG. 1 is a perspective view of the dining arm chair of my design;
FIG. 2 is a top view of the dining arm chair;
FIG. 3 is a front view of the dining arm chair;
FIG. 4 is right side facing view of the dining arm chair (the left side facing view, being a mirror image of the right side facing view, is not shown);
FIG. 5 is a back view of the dining arm chair; and,
FIG. 6 is a bottom view of the dining arm chair.
The dashed lines shown on the bottom of the seat in FIG. 6 are understood to represent stitching.