eg.

Year 1 you have 1 Jan, 1 July and 1 Dec.

Year 2 you have only 1 Dec.

So you add two extra points for year 2, 1 Jan and 1 July calculating the y value based on a straight line between 1 Dec (year1) and 1 Dec (year 2). (I assume you have a line chart - if it is just points then you have no need to calculate a y value for the padding, just use the same value of y that the x-axis is using to cross the y-axis)