One tricky thing with line graphs is that the dots will be connected in the order that they are in the data table so you may need to sort them by the attribute you’re using on the X axis.
One other thing to consider is if you have several series that you want plotted as separate lines, then you need to group them by some attribute that defines the series. You can do that by dragging that attribute to the left side of the table all the way until you see a place that will light up yellow and you can drop to group them by that attribute