'pixels' isn't an allowable argument for the x value position...it's a char() array string array and that's confusing the input parser inside scatter() -- it's trying to interpret the character string argument as one of the trailing optional arguments like the colorspec or the like, but as the documentation says, the x, y data have to be supplied before any of those.
scatter() also MUST have both x and y data arguments; you can't just give it a y vector like plot(). It will accept a categorical array as the X axis value, but you'll have to define it for the six categories to which each of the y variable values belongs to do so.
