What do you expect? A single line or multiple line series?
Your line looks the way it does because your X values are not linearly increasing. Instead, they increase and decrease in value, which results in your line looking the way it does. The line is drawn in the order your points specify, the first to the second to the third, etc.
Perhaps you don't want a line. Perhaps you just want the data points. In that case, add a linespec to your plot command, or use scatter. If you do want the line, then you'll have to sort your data.
C=0.0893;
a=1.5532;
n=2.9718;
x=[2.21 2.24 2.46 2.58 2.12 2.27 3.12 3.7 3.08 2 1.47 2.43 2.16...
3.48 1.61 0.93 1.74 2.3 2.38 2.33 2.38 2.06 2.76 3.5 0.9 2.33...
3 4.31 4.21 2.27 2.16 3.95 2.08 3.42 2.81 3.9 4.87 5.32 3.93...
4.12 5.17 5 5.54 3.11 4.9 5.22 3.48 3.07 2.88 2.32 2 2.7 4 0.33...
3.62 3.47 2 1.92 1.8 1.82 3.39 3.18 3.83 4.11 3.655 2.55 2.22 3.77 2.96];
y=C*(x+a).^n;
% Set LineSpec
plot(x,y,'o')
% Use scatter
figure
scatter(x,y)
% Sort x data first
x=sort([2.21 2.24 2.46 2.58 2.12 2.27 3.12 3.7 3.08 2 1.47 2.43 2.16...
3.48 1.61 0.93 1.74 2.3 2.38 2.33 2.38 2.06 2.76 3.5 0.9 2.33...
3 4.31 4.21 2.27 2.16 3.95 2.08 3.42 2.81 3.9 4.87 5.32 3.93...
4.12 5.17 5 5.54 3.11 4.9 5.22 3.48 3.07 2.88 2.32 2 2.7 4 0.33...
3.62 3.47 2 1.92 1.8 1.82 3.39 3.18 3.83 4.11 3.655 2.55 2.22 3.77 2.96]);
y=C*(x+a).^n;
plot(x,y)
