In this demo, increasing x is indicated by red, decreasing x by absence of red, increasing y by blue, and decreasing y by absence of blue. Consequently, when x and y are both increasing, the red and blue will overlap in a purple section of the curve. The curve will be white where both x and y are decreasing.
You can use these colors to find critical points. Any point at which the curve changes from red to white or purple (or vice versa) is a critical point for x(t) and any point at which the curve changes from blue to white or purple (or vice versa) is a critical point for y(t).