Replacing matrix values with a cumulative time step

I want to make a graph by plotting a variable (LWP data) against time. I have the data in a matrix format so I am assuming that it needs to be plotted against another matrix (time).

Below you can see the format of my data, each day has a different number of measurements.

My idea is to duplicate the LWP data matrices for every day and then replace the cells as time step values, e.g. increasing every 2.87 seconds. However, I am struggling with this. I can create a vector with increasing time step values but I don't know how to convert this into a matrix form.

Each day's time matrix needs to have the same number of rows as the LWP data.

'LWP data'  
47x639 single   
47x1429 single   
47x869 single   
47x1429 single   
47x1420 single  
47x1132 single

Thank you very much!