In this paper, we study the probability distribution of the observable $s = (1/N)sum_{i=N-N+1}^N x_i$, with $1 leq N leq N$ and $x_1<x_2<cdots< x_N$ 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 $N = kappa , N$ (with $0 < kappa leq 1$) rightmost particles. It interpolates between the position of the rightmost particle (in the limit $kappa to 0$) and the full center of mass (in the limit $kappa to 1$). We show that, for large $N$, $s$ fluctuates around its mean $langle s rangle$ and the typical fluctuations are Gaussian, of width $O(N^{-3/2})$. The atypical large fluctuations of $s$, for fixed $kappa$, are instead described by a large deviation form ${cal P}_{N, kappa}(s)simeq exp{left[-N^3 phi_kappa(s)right]}$, where the rate function $phi_kappa(s)$ is computed analytically. We show that $phi_{kappa}(s)$ takes different functional forms in five distinct regions in the $(kappa,s)$ plane separated by phase boundaries, thus leading to a rich phase diagram in the $(kappa,s)$ plane. Across all the phase boundaries the rate function $phi(kappa,s)$ 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.
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