Robustfit will not it to data on graph

Why wont the robustfit fit to my data points? The data plots are obviously showing most along the y-direction (attached file). My code is as follows:

mask = (x > 0) & (y > 0.004);

[b,stats]=robustfit(x(mask),y(mask));

x1=[min(x(mask)),max(x(mask))];

y1= b(1)+b(2)*x1;

hold on

plot(x(mask),y(mask),'.b');

plot(x1,y1,'-r');