You are absolutely right in the assumption that the chart must draw a vertical line when the X values are the same and Y values vary. However, the type of the produced line depends on the used axis scales and groups in the graph. I guess the X axis of your report uses a Category scale, which creates a separate slot for each group member defined by the graph's category group. I'm sure it sounds a bit vague, so I will try to explain it below in details:
I will assume that the data passed to the graph contains just two fields: X and Y to define the horizontal and vertical coordinates per each point. In the current scenario the graph item uses the category groups to divide the X axis in slots. When the Grouping property is set to =Fields.X, all the X entries with equal values will be grouped in a single slot and this slot will have only one corresponding Y value - usually some aggregate like =Sum(Fields.Y). Obviously in this scenario we don't want to group the X data, so the Grouping expression must be set to an expression that produces a new group member per each row in the data source. This is called Detail Grouping and is defined by a Grouping with empty expression.
Once created the detail grouping for the graph's Category group, you will notice that the Category scale is not the best choice for such scenario, because it creates slots per each X instead of putting the constant ones together. In this case a Numerical scale should be used. When replacing the X axis' scale with Numerical one, you also have to define the series' X expression, because the Category scale infers it from the category groups, but Numerical scale doesn't. Therefore the series should have its X expression set as =Fields.X and the produced graph will look as you expect.
I shot a small video demonstrating how to create such a chart using the Graph item. The video can be seen here: https://www.screencast.com/t/wm8cppKrD (please note it has no sound and requires Flash to run). I will also attach the produced .trdx file as a reference. I hope the detailed explanations will help you understand how the Graph groups its data and what are the differences between various axis scales. Of course, if you need further assistance, please do not hesitate to ask.
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