I used to be able to cut and paste excel line charts into illustrator and to apply stroke formatting to the lines there. These days - on line charts with lots of data points this results in an outlined stroke (i.e. a fill with no stroke). This also appears when exporting as a pdf and opening in illustrator. This applies to charts with no effects applied.
Sometimes I can get around this by opening in open office and exporting as a pdf, or saving as an xls file and opening on an older version of excel - but this regularly fails (either it won't open or won't draw the chart).
Does anyone have a clue how to get back to single lines rather than outlines?