Here's an image for what the "step" modes could look like. Note that this is for "worst case" data scenarios, since most users of the chart would order the points correctly if they wanted to use the ScatterLine series.

Current chart example (see the points that are being plotted here): https://dojo.telerik.com/unEFIVOM/2
Step style mockups: https://i.imgur.com/SJAqgvq.png

I'm also looking for this functionality. The exact same code currently used to draw a ScatterLine series could be used for the StepFirst/StepLast lines, but instead of drawing the straight line it would draw the Y component of the vector first, then X component (StepFirst). Or it would draw the X component of the vector first, then the Y component (StepLast). 

Even though Scatter series allows specifying points in any order the above rule can still apply; in this example (https://dojo.telerik.com/unEFIVOM/2) the line would move in the -X direction from the 3rd point to the 4th, the same as it does with the current sloped line.

I would need this too!

To clarify - supporting a step mode should include 'step first' and 'step last', where the trace changes to the new value at the new time or the previous time.

Please do this! Other trend packages can.