Why the Total Revenue curve of a competitive firm faces a straight line passing through origin?

A competitive firm sells its output at. the uniform price. The price or AR is constant and MR is also constant which is equal to AR. The Total Revenue is the sum total of MR corresponding to different levels of output. Since, MR is constant, TR increases at a constant rate. Thus, TR curve is a straight line. It passes through the origin because when sale is zero, TR is also zero.