I have assumed that the signal has only 1 peak. In case of multiple peaks, the "find(P == max(P), 1)" command will return an array of indices, each corresponding to a different maximum value in the signal.
I suppose shifting the peak signal value to the center, can be done in two ways depending on where the peak is occurring in the signal.
If the peak occurs before the mid point, then find the value of the signal at as many points before the very first one as the peak is from the mid point. Then concatenate these values to the beginning of the signal array. For the sake of demonstrating I have assumed that these values are zero and come up with teh following:
new_P = [zeros(length(P)/2 - find(P == max(P), 1, 'first'), 1); P];
Similarly if the peak occurs after the midpoint:
new_P2 = [P; zeros(find(P == max(P), 1, 'last') - length(P)/2, 1)];
If, on the other hand you want to change the axes range for plotting the signal, such that the peak appears at the centre, then you can find the index of the peak value using:
find(P == max(P), 1, 'first');
Then, use this value to set the axes range and aspect ratios as shown in the following document: