Percent increase in mortality for 1oC increase in temperature

Hello everyone.

I have a timeseries of daily values of temperature and number of deaths (see attached Daily_Data_MoT.xlsx). I am trying to find a way to calculate percent increase in mortality per 1oC increase above a certain heat threshold. Eg. if the threshold is 30oC, there is a 2.1% increase in mortality per 1oC increase in temperature (31oC, 32oC, ...).

Eventually (hopefuly) I will be able to produce a table like this:

For the time being I am doing

data = readtable('C:\PhD\ELSTAT\Data\Daily_Data_MoT.xlsx');

%% Calculate the first derivative
data = sortrows(data, 8);
percent_change = diff(data.Daily_Deaths) ./ data.Daily_Deaths(1:end-1) * 100;

data(1,:) = [];

idx = data.Daily_T > 28;

plot(data.Daily_T(idx), percent_change(idx))

Nevertheless I am not sure it works... I would appreciate any help!!

PS. I am on R2019a