The behavior is expected as a line is drawn between two points. When you have one data point there isn't an option to draw a line.
For example draw a point mark on the chart data point. so when you have one data item, a circle will be drawn to indicate that there is a value. I have attached the files from my project. The MAUIChartCustomization.zip contains 3 files the MainPage.xaml, MainPage.xaml.cs and MyCustomChart.cs
And in order to draw the circle you will need a platform behavior. The PointRenderer.zip contains the PointRenderer.cs file which must be added inside the Platforms/Android folder