In this paper, we study the probability distribution of the observable s = (1/N )representing the ordered positions of N particles in a 1d one-component plasma, i.e., N harmonically confined charges on a line, with pairwise repulsive 1d Coulomb interaction |x i − x j |. This observable represents an example of a truncated linear statistics -here the center of mass of the N = κ N (with 0 < κ ≤ 1), rightmost particles. It interpolates between the position of the rightmost particle (in the limit κ → 0) and the full center of mass (in the limit κ → 1). We show that, for large N , s fluctuates around its mean s and the typical fluctuations are Gaussian, of width O(N −3/2 ). The atypical large fluctuations of s, for fixed κ, are instead described by a large deviation form P N,κ (s) exp −N 3 φ κ (s) , where the rate function φ κ (s) is computed analytically. We show that φ κ (s) takes different functional forms in five distinct regions in the (κ, s) plane separated by phase boundaries, thus leading to a rich phase diagram in the (κ, s) plane. Across all the phase boundaries the rate function φ(κ, s) undergoes a third-order phase transition. This rate function is also evaluated numerically using a sophisticated importance sampling method, and we find a perfect agreement with our analytical predictions.