To generate a trend line, the algorithm takes into account the round date. Probably you have several rounds kept with the same date. Please edit the round date, click here (iOS) or here (Android) to see how to do it.
Also, you will need a minimum of 5 rounds played in the past 6 months to generate a trend line.
On the first screen, you can see the graphic when you have less than 5 rounds played in the last 6 months. On the second screen, the graphic shows a player with 5 or more (20 max.) rounds played in the last 6 months.