R, 239 bytes

function(s){K=table(el(strsplit(gsub(" ","",s),"")))
L=sum(K|1)+1
M=(M=max(K))+-M%%5+1
m=matrix(" ",L,M)
m[2:L,M]=names(K)
m[1,M-g]=paste0(g<-c(1,seq(5,M,5)),"-")
m[1,]=format(m[1,],j="r")
for(X in 2:L)m[X,M-1:K[X-1]]="X"
write(m,1,L,,"")}

Try it online!

table does the heavy lifting here, uniquifying the characters, sorting them, and returning their counts.

Everything else is just ensuring the offsets are right for printing, which is the "real" work of an ascii-art challenge.

Thanks to @dylnan for pointing out a bug.

R, 239 230 bytes

K=table(el(strsplit(gsub(" ","",scan(,"")),"")));m=matrix(" ",L<-sum(K|1)+1,M<-(M=max(K))+-M%%5+1);m[2:L,M]=names(K);m[1,M-g]=paste0(g<-c(1,seq(5,M,5)),"-");m[1,]=format(m[1,],j="r");for(X in 2:L)m[X,M-1:K[X-1]]=0;write(m,1,L,,"")

Try it online!

table does the heavy lifting here, uniquifying the characters, sorting them, and returning their counts.

Everything else is just ensuring the offsets are right for printing, which is the "real" work of an ascii-art challenge.

Thanks to @dylnan for pointing out a bug.

Thanks to @rturnbull for the scan approach, dropping 2 bytes.

R, 221 217 bytes

function(s){K=table(el(strsplit(gsub(" ","",s),"")))
L=sum(K|1)
M=(M=max(K))+-M%%5+1
m=matrix(" ",L+2,M)
m[1:L+2,M]=names(K)
m[1,M-g]=g=c(1,seq(5,M,5))
m[2,M-g]="-"
for(X in 1:L)m[X+2,M-1:K[X]]="X"
write(m,1,L+2,,"")}

Try it online!

table does the heavy lifting here, uniquifying the characters, sorting them, and returning their counts.

Everything else is just ensuring the offsets are right for printing, which is the "real" work of an ascii-art challenge.

R, 239 bytes

function(s){K=table(el(strsplit(gsub(" ","",s),"")))
L=sum(K|1)+1
M=(M=max(K))+-M%%5+1
m=matrix(" ",L,M)
m[2:L,M]=names(K)
m[1,M-g]=paste0(g<-c(1,seq(5,M,5)),"-")
m[1,]=format(m[1,],j="r")
for(X in 2:L)m[X,M-1:K[X-1]]="X"
write(m,1,L,,"")}

Try it online!

table does the heavy lifting here, uniquifying the characters, sorting them, and returning their counts.

Everything else is just ensuring the offsets are right for printing, which is the "real" work of an ascii-art challenge.

Thanks to @dylnan for pointing out a bug.

R, 221 bytes

function(s){s=el(strsplit(gsub(" ","",s),""))
K=table(s)
L=sum(K|1)
M=(M=max(K))+-M%%5+1
m=matrix(" ",L+2,M)
m[1:L+2,M]=names(K)
m[1,M-g]=g=c(1,seq(5,M,5))
m[2,M-g]="-"
for(X in 1:L)m[X+2,M-1:K[X]]="X"
write(m,1,L+2,,"")}

Try it online!

table does the heavy lifting here, uniquifying the characters, sorting them, and returning their counts. Everything else is just ensuring the offsets are right for printing.

R, 221 217 bytes

function(s){K=table(el(strsplit(gsub(" ","",s),"")))
L=sum(K|1)
M=(M=max(K))+-M%%5+1
m=matrix(" ",L+2,M)
m[1:L+2,M]=names(K)
m[1,M-g]=g=c(1,seq(5,M,5))
m[2,M-g]="-"
for(X in 1:L)m[X+2,M-1:K[X]]="X"
write(m,1,L+2,,"")}

Try it online!

table does the heavy lifting here, uniquifying the characters, sorting them, and returning their counts. 

Everything else is just ensuring the offsets are right for printing, which is the "real" work of an ascii-art challenge.