Fluctuation.

From my merchanting experience that graph looks pretty normal to me, it happens to alot of items. A huge price gain often caused by investers and merchanting clans followed by a huge fall when they sell out to get their profits. That doesn't look like it's been changed by jagex but obviously some items have been.

I don't think that the graph was fixed in any way. It's quite common for items, especially discontinued items to have jumpy price changes in a short period of time.
Although i do believe that JaGex do change prices when needed.

what im more surprised than this is the strange fact that cheap holiday items like santas still have a market. I thought after the death of staking and item lending noone even buys them anymore.

Ok, this may not be normal but it is not jagex interfering with the prices (in this example at least). The cause of this wild fluctuation is caused almost entirely by merchanters. They lock their sights on an item that has a market (shown by small fluctuations) and start buying like crazy. After a few days of this merchants with less money will start buying, to try to make a huge amount of profit. When the larger investers start getting worried that the bubble will burst, they'll start selling out all their santa hats causing a slowdown in the price rise (note the top of the that fluctuation). Once the larger merchants have sold out the smaller merchants notice this and frantically try to sell their hats causing a huge decline in price that lasts for days.
Jagex does control some prices, such as items that have just come out or the expensive rares(phats) but I doubt that Jagex currently controls the prices of santas.