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Is it clear enough to observe the interaction between z and x from these bubble plots? If there is no interaction between x and z, the size of bubbles will increase at the same rate while either x or z is fixed. But when interaction exists, the rate of increasing will be different. R code: sq = 1:10 x = rep(sq, 10) z = rep(sq, each = 10) y = c(outer(sq, sq, function(x, z) 10 + 1 * x + 0.5 * z + 0.5 * x * z + runif(1))) symbols(x, z, y, bg = "red", fg = "yellow", main = "y = 10 + x + 0.5 ...