gnuplot missing data with expression evaluation

I want to use the plot command in gnuplot with expression evaluation, i.e.

plot "-" using ($1):($2) with lines
1 10
2 20
3 ?
4 40
5 50开发者_StackOverflow社区
e

But I want it to ignore the missing data "?" in such a way that it connects the line (and doesn't break it between 2 and 4).

I tried set datafile missing "?", but in agreement with the online-help it does not connect the lines. The following would, but I cannot use expression evaluation:

plot "-" using 1:2 with lines
1 10
2 20
3 ?
4 40
5 50
e

Any ideas how to connect the lines and use expression evaluation?

Two column data

If you set up a data file Data.csv

1 10
2 20
3 ?
4 40
5 50

you can plot your data with connected lines using

plot '<grep -v "?" Data.csv' u ($1):($2) w lp

More than two column data

For more than two columns you can make use of awk.
With a data file Data.csv

1 10 1
2 20 2
3 ?  3
4 40 ?
5 50 5

you can run a script over the data file for each plot like so:

plot "<awk '{if($2 != \"?\") print}' Data.csv" u ($1):($2) w lp, \
     "<awk '{if($3 != \"?\") print}' Data.csv" u ($1):($3) w lp

A reference on scripting in gnuplot can be found here. The awk user manual here.