dear R users,
my data looks like this

         PM10       Ref   UZ     JZ         WT   RH   FT   WR
1   10.973195  4.338874 nein Winter   Dienstag   ja nein West
2    6.381684  2.250446 nein Sommer    Sonntag nein   ja  Süd
3   62.586512 66.304869   ja Sommer    Sonntag nein nein  Ost
4    5.590101  8.526152   ja Sommer Donnerstag nein nein Nord
5   30.925054 16.073091 nein Winter    Sonntag nein nein  Ost
6   10.750567  2.285075 nein Winter   Mittwoch nein nein  Süd
7   39.118316 17.128691   ja Sommer    Sonntag nein nein  Ost
8    9.327564  7.038572   ja Sommer     Montag nein nein Nord
9   52.271744 15.021977 nein Winter     Montag nein nein  Ost
10  27.388416 22.449102   ja Sommer     Montag nein nein  Ost

.

.

.

.

til 200


I'm trying to make a linear regression between PM10 and Ref for each of the
four WR, I've tried this:
plot(Nord$PM10 ~ Nord$Ref, main="Nord", xlab="Ref", ylab="PM10")
but it does not work, because "Nord cannot be found"
what was wrong? how can I do it? please help me

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