How can I make this line chart with smooth line in Tableau

Question

I am new to Tableau. I want to make a line chart like the one shown here (Sorry for the quality of the image the app I took it from doesn't allow screenshots due to security policy):

The difference is that I want to show the balance going in minus.

I was able to do it by making the account balance discreet and dimension and making the date continuous.

I got this result:

Can anyone help me achieve the result I want?