<<tagName>>
<$list filter="[tag<tagName>tag[Journal]!sort[title]]" />
</$set>
\end
Note that there are single < and > around tagName in the filter instead of [ and ].
If you have something like this:
<$set name=tagName value={{!!title}}>
<<TagHistory>>
</$set>
then you could use $(tagName)$ inside the TagHistory macro.
This explaination may be missing some details, but the general difference between <<tagName>> and $(tagName)$ is when you use <<tagName>> it treats it like a macro and inserts the wikitext which is then evaluated, if you use a set widget outside a macro and then use $(tagName)$ inside the macro than the wikitext that defines tagName is evaluated before being passed into the macro.