time  site    rack    sensor    reading
0             rack0   sensor0   
1             rack1   sensor1   1.25
2     null    rack2   sensor2   2.5
10    site10  rack10  sensor10  12.5
11    site11  rack11  sensor11  13.75
12    site12  rack12  sensor12  
13    site13  rack13  sensor13  16.25
14    site14  rack14  sensor14  17.5
15    site15  rack15  sensor15  18.75
16    site16  rack16  sensor16  
17    site17  rack17  sensor17  21.25
18    site18  rack18  sensor18  22.5
19    site19  rack19  sensor19  23.75
3     site3   rack3   sensor3   3.75
4     site4   rack4   sensor4   
5     site5   rack5   sensor5   6.25
6     site6   rack6   sensor6   7.5
7     site7   rack7   sensor7   8.75
8     site8   rack8   sensor8   
9     site9   rack9   sensor9   11.25
