Plotting psychological round numbers

This simplest way to achieve this is to adjust the chart settings. First two screenshots below show how to adjust the setting named "Custom Interval", located within the "Price axis" section of the chart settings. I have included two examples in the screenshots below. One for an interval of 25 and another for the interval of 50.

If you did not know this setting was available, be sure to check out Part Two of our master class on chart settings:

https://www.hahn-tech.com/thinkorswim-chart-settings-tutorial/

Another very simple method is to create a custom chart study which plots the levels based on your own user inputs. The following section of code plots four different lines which you manually enter in the user input for each level:

input levelOne = 125;
input levelTwo = 150;
input levelThree = 175;
input levelFour = 200;
plot firstLevel = levelOne;
firstLevel.SetDefaultColor(Color.GRAY);
plot secondLevel = levelTwo;
secondLevel.SetDefaultColor(Color.GRAY);
plot thirdLevel = levelThree;
thirdLevel.SetDefaultColor(Color.GRAY);
plot fourthLevel = levelFour;
fourthLevel.SetDefaultColor(Color.GRAY);

Yes, it is possible to create a more complex chart study which is able to automatically adjust those levels based on current price. But the complexity of that solution is way beyond the scope of solutions I can provide free of charge in the Q&A Forum.