Riemann Integrability and Jordan Measure It can be proven that if a function, $f : [a,b] \rightarrow \mathbb{R}$ is Riemann-integrable then its graph is measurable and has Jordan measure $0$. Is the converse, for a function true?
That is; 
If for a function $f : [a,b] \rightarrow \mathbb{R}$, the set $\{(x,f(x)) | x \in [a,b]\}$ is Jordan measurable with measure $0$, does it follow that $f$ is Riemann-integrable.  

Initially I was going to answer your question in the affirmative but I really could not find any flaw in the answers already given so I stopped for a while to figure out why I wanted to answer in the affirmative. This is what I found out. 

Let us start with the following theorem

Theorem: Let $A$ be a non empty and bounded subset of $\mathbb{R} \times \mathbb{R} $. Then $A$ is Jordan measurable if and only if the boundary of $A$ has Jordan measure $0$.

And Riemann integration is intimately linked with Jordan measure via the following theorem

Theorem: Let $f:[a, b] \to\mathbb{R} $ be a non-negative function and let $$A=\{(x, y) \mid x\in[a, b], 0\leq y\leq f(x) \} $$ be its subgraph (area below the graph). Then $A$ is Jordan measurable if and only $f$ is Riemann integrable on $[a, b] $ and further Jordan measure of $A$ is equal to the Riemann integral $\int_{a}^{b} f(x) \, dx$.

I was well aware of these two theorems stated above and then from these one can conclude that

The Riemann integral of $f$ over $[a, b] $ exists if and only if the boundary of subgraph of $f$ is of Jordan measure $0$.

The boundary of subgraph of $f$ has four parts: two ordinates at $a$ and $b$ of lengths $f(a) $ and $f(b) $, part of $x$ axis from $a$ to $b$ and finally the graph of $f$. The first three parts have Jordan measure $0$ and hence it follows that the Riemann integral of $f$ over $[a, b] $ exists if and only if the graph of $f$ is of Jordan measure $0$.
This is wrong!! The subtle flaw in the argument of above paragraph is about the fourth component of the boundary of the region below graph of $f$. It may not just be the graph of $f$ but rather include more points if the function is too discontinuous. Thus if $f$ is the characteristic function of rationals on $[0,1] $ the whole subgraph of $f$ is the boundary and it has outer Jordan measure $1$. The graph of $f$ here is still of Jordan measure $0$. 
