Ethereum Network Difficulty Chart
First thing you need it to get proper data. Here is the link where you can download csv data.
Open downloaded csv file in Excel.
Highlight A and C columns.


Now you have ready Ethereum Network Difficulty Chart and you can move on with your personal edits. You may need to add trendline.
Template
You can download the Template here – Download
Further reading: BitCoin mining calculator Chart with two trendlines Correlation chart