Basically, decimation algorithms reduce number of samples. The length of the vector is smaller, but the time vector has greater dt.
I'm not sure what result do You want to get.
While You decimate the signal, You will get the same shape of the curve, with the smaller number of samples.
If you want to decimate signal, and keep the old dt (fs) (compress signal in time domain ), then You should add zeros at the end of the vector.
But I don't think that is a good idea.