library(readr)

df <- read_csv("yamagata.csv", locale = locale(encoding = "SHIFT-JIS"))

df$`/項目` <- NULL
df$調査年 <- as.numeric(sapply(df$調査年, gsub, pattern="年度", replacement = ""))
df <- df[order(df$調査年),]

sakatashi <- df[df$地域 == "山形県 酒田市",]
sakatashi$地域 <- NULL

sakatashi2 <- na.omit(sakatashi[,c("調査年", "#A05101_人口増減率【％】")])
plot(sakatashi2$調査年, sakatashi2$`#A05101_人口増減率【％】`,
     xlab = "年度", ylab = "人口増減率【％】", type = "b")