line color

I have an XY chart using dates for the X. The Y axis is made up from a
column of equations. When I create the chart, the lines that connect the
data points don't show up. I have been into the plot format and tried making
the lines come up in different colors, but nothing seems to work. However,
when I make a column that is equal to just the value of the other cells and
use this new column to graph the chart, the lines show up, but the dates
change to numbers between 0 and 12 on the chart. How do I make the lines
show up without changing the any of the data?