Ok, almost have it. The dates are showing up properly now, had to use the x-axis format in the chart. My SQL now looks like this:
SELECT [dates]-weekday([dates])
, Sum([press500]) AS [500 Tonne]
, Sum([press600]) AS [600 Tonne]
, Sum([press1000]) AS [1000 Tonne]
FROM [Tmp]
GROUP BY...