I’m don’t know how energy contracts work in Finland, but in Germany you usually have a fixed price per kw/h. That price may change frequently, but it has to be announced and you have the right to cancel the contract each time.

The graph OP showed looks like the price development on the spot market, that’s where energy providers buy gas short-term, apart from their long-term contracts. Spot-market-gas is naturally more expensive than the long-term one. That price may also be very unstable, as for example an unexpectedly cood winter week among several regions/contries can let it hike up pretty drastically.

AFAIK, this short-term price is an option for the private consumer as well. It has the advantage of being much cheaper most of the time when demand is low/normal but the disavantage OP shows here.