If your "year" variable (is it a field or parameter?) is not a date datatype, then ShowMe won't recognize it as valid for a time-series. If it is a field, you can change the datatype (right click the field in the Data Window under Dimensions and select "Change Data Type" --> "Date". If that doesn't work, you might have to create a calculated field, something like: DATE(STR(year) + "-01-01")
Even if ShowMe does not give the option of having a line chart, it can still be accomplished by manually creating a line chart. Once year is on the columns shelf, change the Mark type to line on the Marks card. Then, place the measures on the rows shelf. If you want multiple measures with overlapping lines, use the measure names/values fields.
Hope this helps!
Thanks to both of you. Tracy, that did the trick! I tried your version Joshua,,, seems like it should work all right, but it didn't. I think v8 may still be a little buggy. Again thanks to both. John