Error: "Invalid first data argument" (when attempting to plot)

I am trying to create a scatter plot of: z = ( sin(xy) )2 , where x and y are both independent variables and each point is shown as a square with a defined color (based on it's value). Whenever I attempt to run the code I get the "Invalid first argument" error and it directs me to line 14: (plot(x,y,'s','c')). I honestly have no idea what this means. I have been using MATLAB for only a few months now so practically everything is still pretty new to me. Also, I am supposed to use a for loop to plot each point one-by-one.

if true
 x = -3:0.1:3;
 y = -3:0.1:3;
 z = (sin(x.*y)).^2;
 for i = 1:length(z)
     if z(i) < 0.1
         plot(x,y,'s','b');
     elseif z(i) > 0.1 && z(i) < 0.5
         plot(x,y,'s','c')
     elseif z(i) > 0.5 && z(i) < 0.75
         plot(x,y,'s','g')
     elseif z(i) > 0.75 && z(i) < 0.95
         plot(x,y,'s','r')
     else
         plot(x,y,'s','b')
     end
 end
end

Answer 1

[KSSV]

Hello what is 's' in the plot command? 's' has no meaning in plot. You have to follow as below.

if true

 x = -3:0.1:3;
 y = -3:0.1:3;
 z = (sin(x.*y)).^2;
 for i = 1:length(z)
     if z(i) < 0.1
         plot(x,y,'.b');
     elseif z(i) > 0.1 && z(i) < 0.5
         plot(x,y,'.c')
     elseif z(i) > 0.5 && z(i) < 0.75
         plot(x,y,'.g')
     elseif z(i) > 0.75 && z(i) < 0.95
         plot(x,y,'.r')
     else
         plot(x,y,'.b')
     end
 end
end

[Kevin Steerman]

When I used the "help plot" function, it said that 'S' would creat square data points. And by looking at the edited code you posted, since my x and y values are multi-column vectors, by using the 'b' (as I did) it was trying to do the integer procedure because I did not specify my x and y datas are vectors. Is that statement correct?

Also, thank you so much for your response!

[KSSV]

'S' must be a string; which specifies marker and color.