You're right, there are a few different ways to go on this one. As prices can fluctuate from $1-$300 within seconds, having a general bar graph from $0-$500 would look dumb. It needs to show changes in relation to where it is at this time, like a stochastic graph irl stocks.
Perhaps an easy way to show fluctuations is if the graph is constantly calculated in relation to it's current price. So if it shows $1, and it's been that way for a while, then it would have a flat line. If it was $100, but just rose from $50, the bar would be twice the size as it were a minute ago.
I think a 5 minute increment would have to be a maximum, as these games usually only last 20 mins or less, and anything further is irrelevant. Perhaps an update twice per 'day' - at noon and midnight would suffice? This would be an update every 1 minute & 20 seconds? Enough to show how things have changed, spot trends between day/night cycles, and allow the interesting interaction where people can purposely manipulate the market around noon/midnight to try to skew these graphs.
I think it's a great idea, unless the devs want us to keep track of this ourselves, and would thus increase a player's "skill" by following the trends with the naked eye while also focusing on other issues.
