To make a graph, you need more than one data point. If your data is constantly being overwritten, then you have only one data point, which makes for a pretty useless graph! If you can store the data in successive columns (or rows), so that day 1 value is column 1, day 2 is column 2 etc, then you can easily create a graph based on the range of columns you have saved. Insert a graph from the Insert tab (assuming Excel 2007 or later, I'm looking at 2010). Right click on the graph space and Select Data, ensuring that you include the column title (e.g. date) as well as the data value. Voila. You can play around with a sliding week / month / year range if you like, once you have data...
... but why not just go to one of the many online resources that give you huge amounts of graphs and other data on every stock / share / fund / bond in the world - free! e.g. Hargreaves Lansdown -
https://www.hl.co.uk/shares
SD