Hi Mel,

To plot or extract every 20th value from your dataset, you can use the INDEX function in Microsoft Excel. Here are the steps:

1. Select a new column where you want to extract every 20th value.
2. In the first cell of the new column, enter the following formula:
   Formula:

   =INDEX(A:A,20*(ROW()-1)+1) 

3. Replace "A:A" with the column where your time values are located.
4. Drag the formula down to the end of your dataset.

This formula will extract every 20th value from your time column. You can then copy and paste the extracted values into a new sheet to plot them.

Alternatively, you can use the same formula to plot every 20th value directly in a chart. Here are the steps:

1. Select the time and concentration columns.
2. Go to the Insert tab and select a chart type that you prefer.
3. Right-click on the chart and select "Select Data".
4. Click on "Add" to add a new series.
5. In the "Series values" field, enter the following formula:
   Formula:

   =INDEX(B:B,20*(ROW()-1)+1) 

6. Replace "B:B" with the column where your concentration values are located.
7. Click "OK" to close the "Select Data" window.

This will plot every 20th value from your concentration column in the chart.