The data shown for ISF.L is inconsistent.
The price of an entire ISF share is in the range of 500-700 gbp, as you can see in the London Stock Exchange website (https://www.londonstockexchange.com/exchange/prices-and-markets/ETFs/company-summary/IE0005042456IEGBXEUET.html).
Now, you can buy a fraction of the stock (1/100th for example) and then the reported price would be around 5-7gbp.
The problem from Yahoo is that you are reporting the entire price for some period of time and the 1/100 for others which make the data/graphic jump from 500/700 to 5/7 at any time.
Now you can either report the entire stock price as the LSX does or the 1/100 but you need to report it consistently otherwise is very difficult to use the reported data.
Please, do not hesitate to let me know if you require further clarification or if I can help in anyway.