Excluding data for a linear regression in OpenOffice

[xAXISx]

For my chemistry lab, in order to computer change in temperature for a calorimetry experiment we're suggested to take the line of best fit from the peak temperature onwards (excluding the initial data) and extrapolate to y = 0. For example, here's some of the data I gathered:

A, Trial 2
Time (s) Temp (°C)
5 29.0
10 31.0
15 33.0
20 33.5
25 34.0
30 34.0
40 33.5
50 33.5
60 33.5
70 33.0
80 33.0
90 33.0
120 32.5
150 32.5
180 32.5
210 32.0
240 32.0
270 32.0

Basically, I want to take the line of best fit from 25 seconds on (as this is the peak of the temperature), excluding everything else. I want to put this line on the same graph as the data itself. Is there any way to exclude data when calculating the line of best fit?

 

[Ygggdrasil]

I would plot the data from 25 seconds onward as one data series, fit them, then add the data points from < 25s as a second data series onto the same graph.