Both seem to be line charts and not scatter charts.
General rule: In line charts, the order of data is kept in the chart (from left to right), even if the X-axis includes numbers that are not in ascending order. Exception: If X-axis only includes dates, then points are plotted (from left to right) in ascending order dates.
December is not a date. Therefore, the general rule is kept.
All X-axis data are dates. Therefore, the exception applies.
Excel interprets 17 and 18 not as years but a the days of the month that follows.
Furthermore, all dates are assumed to belong to the current year.
Because of the previous two sentences, the points are displayed (from left to right) in the following order: 17.Jan, 18.Jan, 17.Feb, 18.Feb,...