Forget about the chart for now. A chart is just a way of DISPLAYING data. What you're describing is data mining, which doesn't require a chart.
Here's what you can do... If you have a table of data for your chart like this:
LineID CoorX CoorY
12 5 32
13 16 47
14 23 31
15 42 45
16 67 48
And you have another table with a single pair of coordinates:
FindID FindX FindY
91 23 31
You can use a DLookup() function to find which record in the larger table matches the record in the single-record table:
X = DLookup("[LineID]", "tblChartData", "[CoorX]=" & FindX & " And [CoorY]=" & FindY)
"LineID" represents the Primary Key field for the first table, a field that has a unique value for each record. People generally use an Autonumber field for this sort of thing. The above DLookup() function will return a value of 14, indicating that the record where LineID = 14 is the one that contains the coordinates that match the ones in your table with a single value.
Now you have figured out which record in the chart data is the one that needs to be colored, and you didn't need the chart to do so. You also didn't need to read through each record one at a time - the DLookup() function just needs to know how to filter down it's results. In this case, the CoorX value in the chart table had to match the FindX value in the other table AND the Coor& value in the chart table had to match the FindY value in the other table. Once the DLookup function finds a record that matches those rules, it returns the value for the field you ask for. For more info, check out the Help file on DLookup in Access's Visual Basic Editor.
Hope this helps,