Need help plotting a 2D matrix

Hi All,

I am having difficulties plotting a 2D matrix showing how a particle number distribution changes with time. For this image, the x axis is the date/time, y axis is the particle diameter, and the number concentration for each particle size bin is represented as a color based on a color bar. An example of the type of figure I want to make is shown in the attached jpeg file (2D matrix I want to make).

Based on what I have read in the forum, I have created a XY matrix that shows the number concentration for each particle diameter as a function of time, a time wave (named Date_time_waveX), and a particle diameter wave (named Dp_Ywave). However, when I clicked Windows>New>Image plot (see screenshot (1), I am not able to choose my x wave and y wave (see screenshot (2) and (3)). Basically, I am forced to choose the root folder for my x and y waves but my time waves and particle diameter waves are not showing up under the root folder.

I am not sure what I did wrong and how to proceed. I know it is likely I missed something while going through the forum and the manual but I can't figure out what it is. I would appreciate it if someone can provide some insights as to what I did wrong. I have attached my file. Thanks in advance.