If you want to visualize the trend lines on the discrete data. Then follow the below steps :
Step 1 - Drag those fields in Tableau Desktop
Step 2 - Visualize the data in Bar Chart.
Step 3 - Right-click on the plot there you will see Trend line option just click on that.
So now you will get the desired result.
You will get more details on analytics in Tableau check tableau training.