1. A non-transitory, computer-readable recording medium storing a physical quantity distribution calculation program to be executed by a computer that causes a display to visualize a physical quantity distribution in a target space for numerical analysis, the program causing the computer to perform the processes of: based on positional information on a plurality of grid points defined in the target space and physical quantities obtained respectively at the plurality of grid points by the numerical analysis, grouping the plurality of grid points into groups of neighboring grid points having the same physical quantity; determining the physical quantity to be visualized by each of a plurality of pixels of the visualization device which are used to visualize and display the physical quantities obtained respectively at the plurality of grid points by the numerical analysis and are fewer than the plurality of grid points, the determining process including extracting for the respective pixels, from the groups formed in the grouping process, at least one group including at least one of grid points corresponding to the pixel out of the plurality of grid points, identifying the group including the largest number of grid points among the extracted groups, and determining that the physical quantity at the grid points belonging to the identified group is the physical quantity to be visualized by the pixel, and displaying the pixels on the visualization device, wherein the numerical analysis is electromagnetic field analysis, and the physical quantities are electromagnetic field strength obtained by the electromagnetic field analysis, when the electromagnetic field analysis is performed at a first time point in a period of analysis time during which the numerical analysis is performed and at a second time point in the period of analysis time, the second time point being before the first time point in the period of analysis time, information that contains the physical quantities obtained respectively at the plurality of grid points at the first time point by the electromagnetic field analysis includes, for each of the plurality of grid points, electromagnetic field strength at the first time point, a charge moving direction at the first time point, and a difference value between the electromagnetic field strength at the first time point and electromagnetic field strength at the second time point, and in the grouping process, the program causes the computer to group the plurality of grid points into the groups of neighboring grid points having the same electromagnetic field strength, the same charge moving direction, and the same difference value, which are included in the information that contains the physical quantities.