ggplot2 map without legend

Question

I've made a plot on a map with ggplot. But I've no legend even with specified instructions.

I need to show the correspondence in size point in legend like in the picture below:

I try to have in legend : points (size='graph') and their value . it's the only type of graph, i've encountered this issue.

Here's my data:

data.frame(
       graph = c(2.5, 1.5, 1.5, 1.5, 1.5, 3, 1.5, 1.5, 1.5, 1.5, 1.5, 1.5, 1.5,
                 1.5, 1.5, 1.5, 1.5, 1.5, 1.5, 1.5),
           x = c(690407.686028692, 668622.75462895, 696803.566124092,
                 673062.214891085, 656077.411408432, 683634.893828692,
                 708826.685978697, 666676.936826137, 680202.347931864,
                 685075.304813531, 702433.968175714, 698431.441510561,
                 676293.390383419, 695645.844602817, 687566.815694649, 663013.779537116,
                 646481.359172896, 638110.223907023, 640715.699972469,
                 642273.74623275),
           y = c(6334609.90229255, 6335374.93429776, 6335888.95948295,
                 6336900.98154074, 6337072.93894301, 6338323.39600256,
                 6341716.94405495, 6345012.80734466, 6345548.51942826,
                 6346087.10440564, 6348894.7081605, 6350228.42781669, 6350778.18047204,
                 6353033.14045844, 6353109.41298154, 6354640.69154103,
                 6360042.46765469, 6360058.68711873, 6360425.02344172,
                 6363722.52659089),
         lat = c(49.3508825326633, 49.3553591808786, 49.3583668130435,
                 49.364287756, 49.3652937402597, 49.3726085383319,
                 49.3924542903226, 49.4117211016949, 49.4148520294118,
                 49.4179995462185, 49.434404050505, 49.4421948987342, 49.4454058876811,
                 49.4585744317618, 49.4590197871622, 49.4679600691824,
                 49.499485003012, 49.4995796297469, 49.5017168409091,
                 49.520950318328),
        long = c(6.20203776633165, 6.00634039793282, 6.25949293478261,
                 6.046220748, 5.89364366233766, 6.1411967388374,
                 6.36749845806452, 5.98886081920904, 6.11036165441177,
                 6.15413617086835, 6.3100716969697, 6.27411638818565, 6.07524689130435,
                 6.24909294540943, 6.17651779391892, 5.95595411740042,
                 5.80744085843373, 5.73224167088608, 5.75564706060606,
                 5.76964322829582)
) -> tmp

Code:

# create map to project on #from package Openstreemap 
library(OpenStreetMap)
dfmap <- openmap(c(min(tmp$lat) - 0.15, min(tmp$long) - 0.15),  
                 c(max(tmp$lat) + 0.15, max(tmp$long) + 0.15),
                 type ="osm")

# projection
autoplot(dfmap) + 
  geom_point(aes(x, y), 
             size = tmp$graph, color = "brown3", 
             data = tmp) +
  theme(axis.line = element_blank(),
        axis.text.x = element_blank(),
        axis.text.y = element_blank(),
        axis.ticks = element_blank(),
        axis.title.x = element_blank(), 
        axis.title.y = element_blank())