Cdoug3,
I think this is what you want. I do it in a similar manner to that mentioned by pmover. The following instructions are a bit pedantic. I am presuming X-Y charts, haven't checked for the other types.
1) Select a single data point in the curve; usually have to left click a couple of times on the data point to get a fat + sign and a pop-up with the coordinates for the data point.
2) Right click to get a pop-up menu and select "Format Data Point..."
3) A multi-page (or multi-tab) dialogue box titled "Format Data Point" comes up, select tab "Data Labels".
4) Select the option (left click on radio button) "Show Label" and then click the "OK" button. The y value label shows up close to the data point.
5) Left click twice on the label to select it. The first click shows a black square at each end of the label value (text). With the second click, a box outline appears around the label text.
6) Click on the text in the label box and it can be replaced with whatever text is desired. Click outside the area of the label box to exit the text editing mode.
7) Again left click on the label to select it, then right click for a pop-up menu.
8) Select "Format Data Labels" from the pop-up menu which brings up the multi-paged dialogue box titled "Format Data Labels".
9) Select the tab "Alignment". On the right hand side, a group frame titled "Orientation" shows both a diagrammatic positioner and a counter up/down box (correct name slips my mind right now) for degrees. You can grab a pointer in the diagram box or scroll up or down the degrees from horizontal to orient your text in the label box; then click "OK".
You will likely have to repeat steps 7) through 9) to get the label oriented just the way you want it, i.e. parallel to your curve.
10) Left click twice on the label to get the outline box again (keep trying) and by click-and-hold (grabbing) on the edge of the box, you can move the label and place it as you want it with respect to your curve.
You can do this with each data point for every curve in the graph and have a different orientation for each modified label. Go wild! but things may get messed up if you change the graph afterwards, e.g. resizing, not always but sometimes things reset. I do the label thing as the last mod before printing the graph or copying it as a picture into a word document or powerpoint presentation. I do the latter by selecting the chart, then hold down shift and click Edit on the top menu bar; then select "Copy Picture" from the drop-down menu and choose the "As shown when printed" options. I usually paste special in Word as Picture.
Regards