awk and the data manipulation

histogram.dat has the following structure:

-100 -99 7
-99 -98 8
-98 -97 11
-97 -96 10
-96 -95 4
-95 -94 6
-94 -93 11
-93 -92 13
-92 -91 11

I want to change "histogram.dat" to the below structure:

-100 7
-99 7
-99 8
-98 8
-98 11
-97 11
-97 10
-96 10
-96 4
-95 4
-95 6
-94 6
-94 11
-93 11
-93 13
-92 13
-92 11

 awk '{print $1, $3 ; print $2, $3}' histogram.dat > histogram2.dat