The design is the visual features of the CHAIR shown in solid lines in the drawings, whether those features are of shape, configuration, ornament, or pattern, or are a combination of any of those features. The portions shown in stippled lines form no part of the design. Drawings of the design are included. Figure 1 is an isometric view of the CHAIR; Figure 2 is a right side elevation view thereof; where the left side elevation is a mirror image thereof; Figure 3 is a top elevation view thereof; Figure 4 is a front elevation view thereof; Figure 5 is a back elevation view thereof; Figure 6 is an isometric view of a first variant of the chair shown in Figure 1; Figure 7 is a right side view of the first variant shown in Figure 6, where the left side elevation view is a mirror image thereof; Figure 8 is a top elevation view of the first variant shown in Figure 6; Figure 9 is a front elevation view of the first variant shown in Figure 6; Figure 10 is back elevation view of the first variant shown in Figure 6; Figure 11 is an isometric view of a second variant of the chair shown in Figure 1; Figure 12 is a right side elevation view of the second variant shown in Figure 11, where the left side elevation view is a mirror image thereof; Figure 13 is a top elevation view of the second variant shown in Figure 11; Figure 14 is a front elevation view of the second variant shown in Figure 11; Figure 15 is back elevation view of the second variant shown in Figure 11; Figure 16 is an isometric view of a third variant of the chair shown in Figure 1; Figure 17 is a right side view of the third variant shown in Figure 16, where the left side elevation view is a mirror image thereof; Figure 18 is a top elevation view of the third variant shown in Figure 16; Figure 19 is a front elevation view of the third variant shown in Figure 16; Figure 20 is back elevation view of the third variant shown in Figure 16; Figure 21 is an isometric view of a fourth variant of the chair shown in Figure 1; Figure 22 is a right side view of the fourth variant shown in Figure 21, where the left side elevation view is a mirror image thereof; Figure 23 is a top elevation view of the fourth variant shown in Figure 21; Figure 24 is a front elevation view of the fourth variant shown in Figure 21; Figure 25 is back elevation view of the fourth variant shown in Figure 21; Figure 26 is an isometric view of a fifth variant of the chair shown in Figure 1; Figure 27 is a right side view of the fifth variant shown in Figure 26, where the left side elevation view is a mirror image thereof; Figure 28 is a top elevation view of the fifth variant shown in Figure 26; Figure 29 is a front elevation view of the fifth variant shown in Figure 26; andFigure 30 is back elevation view of the fifth variant shown in Figure 26.