This tool shows all Wales opinion polls which have been published since 1999 (the data comes from the polls page of Professor Roger Awan-Scully’s elections in Wales website).
You can change the timeframe of the plot by changing the dates. It is also possible to choose which question is being asked (Region / Constituency or Westminster election). You can also choose which pollsters to display.
Finally you can also choose whether or not to display the running average. This is the mean of the last three polls / elections. Displaying the mean is prefered to showing a line through all points as it smooths the line and means that the plot is less influenced by outliers.