I've experienced similar issues with date grouping.
Unfortunately, I don't have an answer for you. But I do have a guiding thought to consider. The data that you put in the values section is obviously a currency value. I can also see that the label for each of the columns those values are contained are named with the month and year that they correspond with.
What I can tell you is this: Most programs, and I can imagine this applies the same to Power BI, would not be able to identify what date construct those values belong to without an actual date value to correspond them with. I can almost surely guarantee you that Power BI cannot read your column headers and parse the information out to something more generalized. What I mean is that Power BI cannot look at "Mar18" and determine that the reference is to March, 2018 (even if the program did have that capability, how could it know that your referring to March, 2018 when you could just as easily be referring to March, 18th, XXXX).
In essence, what I am trying to tell you is that for what you want, you're most likely going to need a separate column in your data that identifies the month and year in a date format. In essence, instead of having twelve different columns for the profit for each month, you would have one column labeled Date and one column labeled Profit Amount. The values in date would be (January, 2018, February, 2018, etc.) and it would be important to make sure they are formatted as date value type (you can think of when you're using Excel and you use the drop-down to select Currency or Accounting or Short Date). Then, of course, you would have the other column including the values you already have for each month.
By doing this, you would be able to pull the 'Date' column into the X-Axis and the Profit Amount into the Y-Axis. Now that I think about it, if you are only tracking one year, you could call the date column 'Month' rather than 'Date' and just write the name of the month for each value. If you did this, you wouldn't even need to format the values into a date format. Your X-Axis would just display the different text values not even caring whether they are dates or not.
Possible solutions to consider. My personal issue is when you have more than one years worth of data to display. It becomes quite difficult as Power BI does not offer a simple and friendly solution to split months by years (I hope this is something they aim to improve).
