How can we define the dataType for specific rows in a column????

[conx]

The code i have posted here gives the whole column datatype float but i need the dataType for specific rows(for eg: i want string datatype upto row[7] and float datatype for rows greater than 7)

for (var i = 0; i < cols; i++) {
   colModel = { title: toLetter(i), width: 150, minWidth: 50, dataType: 'float' };
}

Any suggestions?
Thanks in advance