Geometrically, it is a smooth curve which slopes downward from left to right.
Now, if the total liquid money is denoted by M, the transactions plus precautionary motives by M1 and the speculative motive by M2, then
M = M1 + M2. Since M1 = L1 (Y) and M2 = L2 (r), the total liquidity preference function is expressed as M = L (Y, r).