To manually calculate a trend line:
+One method, which is not as dynamic is to right-click the trend line and select "Describe Trend Models". From here the trend line model equation can be copied and then used in a calculated field. However, as data changes (i.e. data is filtered or excluded), the calculated fields would need to be updated. For more information regarding this method, please refer to the following Community post: http://community.tableau.com/message/196021
+The second and more dynamic method is to use table calculations to calculate the trend line and slope. See attached workbook.
+The following link from a Tableau blogger on calculating custom trend lines may be useful: http://interworks.co.uk/blog/linear-regression-trend-lines/
trend line generic.twbx 192.8 KB