annotate , xlab , ylab , scale_color_hue, size, legend

df <- data.frame(case_per_capita=as.vector(apply(mdf[,-48],2,sum) / pref_db$x2017),pop_density=pref_db$x2017/pref_db$size,sign=pref_db$x2017,r=pref_db[,2])

p <- ggplot(df, aes(x=pop_density,y=case_per_capita,size=sign,color=r))
p <- p + xlab("人口密度") + ylab("人口あたり件数")
p <- p + geom_point(alpha=1)
p <- p+annotate("text",label=pref_db[1,3],x=df[1,2], y=df[1,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[11,3],x=df[11,2], y=df[11,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[12,3],x=df[12,2], y=df[12,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[13,3],x=df[13,2], y=df[13,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[14,3],x=df[14,2], y=df[14,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[26,3],x=df[26,2], y=df[26,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[23,3],x=df[23,2], y=df[23,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[27,3],x=df[27,2], y=df[27,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[28,3],x=df[28,2], y=df[28,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[40,3],x=df[40,2], y=df[40,1]+0.1,colour='red',family = "HiraKakuProN-W3")
p <- p+annotate("text",label=pref_db[47,3],x=df[47,2], y=df[47,1]+0.1,colour='red',family = "HiraKakuProN-W3")
# p <- p +scale_color_brewer(palette="Spectral")
# p <- p +scale_color_brewer(palette=rainbow(47))
# p <- p +scale_color_brewer()
p <- p + theme_gray (base_family = "HiraKakuPro-W3")
p <- p + scale_color_hue(name="都道府県",labels=pref_db[,3])
# p <- p + guides(fill = guide_legend(reverse = F,order = 2),label = TRUE)
p <- p + guides(size = guide_legend(title="人口"))

# don't forget to set "color=". otherwise fails to show up.

p <- p + geom_smooth(method = "lm",se=F,color="red",size=1)
# p + scale_colour_manual(values = pref_db[,3])
plot(p)

for the final version, see below.

palette("Alpabet")

df <- data.frame(case_per_capita=as.vector(apply(mdf[,-48],2,sum) / pref_db$x2017),pop_density=pref_db$x2017/pref_db$size,sign=pref_db$x2017,r=pref_db[,2])

# p <- ggplot(df, aes(x=pop_density,y=case_per_capita,size=sign,color=r))

p <- ggplot(df, aes(x=pop_density,y=case_per_capita,size=sign,color=r))

p <- p + xlab("人口密度") + ylab("人口あたり件数")

p <- p + geom_point(alpha=1)

p <- p+annotate("text",label=pref_db[attributes(df[df$case_per_capita > 3 ,])$row.names,3],x=df[attributes(df[df$case_per_capita > 3 ,])$row.names,2], y=df[attributes(df[df$case_per_capita > 3 ,])$row.names,1]+0.1,colour='red',family = "HiraKakuProN-W3")

# p <- p +scale_color_brewer(palette="Spectral")

# p <- p +scale_color_brewer(palette=rainbow(47))

# p <- p +scale_color_brewer()

p <- p + theme_gray (base_family = "HiraKakuPro-W3")

p <- p + scale_color_hue(name="都道府県",labels=pref_db[,3])

# p <- p + guides(fill = guide_legend(reverse = F,order = 2),label = TRUE)

p <- p + guides(size = guide_legend(title="人口"))

# don't forget to set "color=". otherwise fails to show up.

p <- p + geom_smooth(method = "lm",se=F,color="red",size=1)  

plot(p)

df <- data.frame(death_per_capita=as.vector(apply(dmdf[,-48],2,sum) / pref_db$x2017),pop_density=pref_db$x2017/pref_db$size,sign=pref_db$x2017,r=pref_db[,2])

# p <- ggplot(df, aes(x=pop_density,y=case_per_capita,size=sign,color=r))

p <- ggplot(df, aes(x=pop_density,y=death_per_capita,size=sign,color=r))

p <- p + xlab("人口密度") + ylab("人口あたり死者数")

p <- p + geom_point(alpha=1)

p <- p+annotate("text",label=pref_db[attributes(df[df$death_per_capita > 0.07 ,])$row.names,3],x=df[attributes(df[df$death_per_capita > 0.07 ,])$row.names,2], y=df[attributes(df[df$death_per_capita > 0.07 ,])$row.names,1]+0.002,colour='red',family = "HiraKakuProN-W3")

# p <- p +scale_color_brewer(palette="Spectral")

# p <- p +scale_color_brewer(palette=rainbow(47))

# p <- p +scale_color_brewer()

p <- p + theme_gray (base_family = "HiraKakuPro-W3")

p <- p + scale_color_hue(name="都道府県",labels=pref_db[,3])

# p <- p + guides(fill = guide_legend(reverse = F,order = 2),label = TRUE)

p <- p + guides(size = guide_legend(title="人口"))

# don't forget to set "color=". otherwise fails to show up.

p <- p + geom_smooth(method = "lm",se=F,color="red",size=1)  

plot(p)