To create a simple trendline showing 'best-fit' linear regression:
When creating the chart, select the dependant variable. (the data you want on the Y axis) In the example below, you would select the data in B3..B8. This type of data is expressed best with an XY Scatter chart.
Your chart for the above data would look something like this:
To add the Time element, use Chart, Source Data, to select the X axis data. (in this example, A3..A8)
A problem common to experimental data is that it contains some random errors (sometimes called noise). The data you just entered has lots of noise! This results in plots which have a certain amount of scatter. If one is trying to fit the data to a straight line, it is not always obvious how the line should be drawn in. Two different students, drawing in what they consider to be the best fit straight line through the data, could end up with significantly different results.
Fortunately, there is a mathematical function that will unambiguously determine the best straight line through a field of data points, no matter how scattered. This function is called Linear Regression and is used in a number chemistry and biology lab experiments. You don't need to understand how the function works to be able to use it.
Excel has a linear regression function that is very easy to use. Basically all you need to do is to tell Excel where the x-values are, where the y-values are, and where in the spreadsheet you want Excel to print the results. The linear regression output from Excel will be printed just to the right of the xy data-pairs, but it can be printed anywhere.
To get to the Linear Regression menu activate the spreadsheet window, then right click on the data points on the chart. SELECT Add Trendline and under the tab labeled Type. You want to create a linear regression so choose Linear. Finally, select OK.
Now that you have a trendline, you need a display of the data for that trendline. To do this, right click on the trendline and SELECT Format Trendline.
Once you are in this window SELECT the Options tab. At the bottom, check both "display equation on chart" and "display R-squared value on chart". SELECT OK. Note: the data will automatically appear over your chart. It is difficult to read the data. So, click on the data, hold down the mouse button and move it if necessary.
The chart with trendline and calculations should look something like this:
